Why Synology NAS Has Slow Transfer Speeds (& How To Fix It!)

I love my Synology NAS DS220+. It all seems much faster than my old D-Link NAS. The admin console is slick, and files transfer quickly… most of the time. Every so often, I go to upload or download a file and it is sloooow.

Luckily there are often a handful of things to check that will fix this problem quickly.

The most common causes are Wi-Fi or other networking issues (on the NAS or local computer), and also temporary IO issues caused by disk or anti-virus scans. However there are a range of settings to check too, such as your SMB settings.

Recap: How Synology NASes Work

Before drilling into exactly why you might be experiencing slow transfer speeds, I wanted to quickly recap on how Synology NASes work – and how they fit into your home network.

A NAS contains one or more drives. Each drive might contain their own data, or they might use RAID for redundancy purposes. You will then connect your NAS to your home network (via ethernet or Wi-Fi), so that everyone in your household can access these files. Since the NAS is connected to the internet, you can also access your files when you’re out of your home (via Synology QuickConnect, for example).

While this is all great, there are quite a few parts that have to work together to allow you to read and write files to your NAS. When any one of these parts has an issue, you may experience slow transfer speeds.

12 Reasons Your Synology NAS Might Have Slow Transfer Speeds

There are quite a few different reasons why your Synology NAS experiences slow transfer speeds, but the first thing to do is… yes, you guessed it:

Turn It Off And Back On Again! Before trying a dozen different fixes for slow Synology file speeds, firstly reboot your NAS via the admin panel. Also consider restarting your router (or network switch), along with the computer you are currently using. This can fix the issue more times than I’d like to admit!

But assuming you have tried that already (or there seems to be a more serious issue at play), let’s explore other possible causes and fixes.

Check You’re Using The Latest SMB Settings

The way that files are stored and organized on your Synology NAS is a little different to how other operating systems (like Windows or Mac) do it. As a result, a protocol known as SMB (server message block) was created. This is essentially a common interface, allowing a range of different systems to read and write files in a standard way.

But just like other protocols change and adapt (think of Wi-Fi – we now have Wi-Fi 5, Wi-Fi 6 and Wi-Fi 6e), so does SMB. There are three main versions: SMB 1, 2 and 3. SMB 1 is slow and really insecure, leading to Microsoft stopping support of it a few years ago.

However if SMB 1 is somehow enabled and the computer you are using is also somehow trying to use SMB 1, you will get slow transfer speeds. SMB 2 and 3 use pipelining technologies to speed up file transfers, but SMB 1 is a lot more linear in how it reads and writes files.

As a result, double check these settings by heading into your Synology NAS admin panel, launching “Control Panel” then “File Services”. Click “Advanced Settings” and you will see the following:

You should ensure that the minimum SMB protocol is SMB 2 (you do NOT want SMB 1 here), and “Enable Opportunistic Locking” should be ticked too. This will ensure that the faster SMB 2 and 3 protocols will be used going forward.

It Might Be A Client Side (Computer) Issue

While it’s easy to blame Synology for slow transfer speeds, it’s entirely possible that the issue is with your actual computer or device. If it has a dodgy Wi-Fi connection, anything from watching videos to transferring files from your NAS will be slow.

Equally if a virus or malware scan is running, your hard drive might be at full utilization – meaning that there is little ‘room’ left for file transfers.

The first thing I would do is launch Task Manager (or its equivalent on Mac/Linux), and look at whether your CPU, RAM and disk usage is higher than expected. It can also be useful to quickly check the bar graphs on the left sidebar – if one of the graphs is maxed out, that might be your problem.

In the case below, everything looks fairly kosher:

However I would probably click “Disk 1 (C:)” and monitor that for a little longer, because it was briefly maxed out. If it keeps getting maxed out, I would try and work out what is causing that – because that will likely be the cause of your slow file transfers.

Secondly, I would double check my internet connection. I would run a speed test, and also try and watch some videos on YouTube. If these are slow or laggy, it might indicate a bad Wi-Fi or ethernet connection – something which would definitely result in slower file transfer speeds with your Synology NAS.

Fixing this problem can unfortunately be a case of trial and error. Assuming you are a long distance from the router (and using Wi-Fi), moving closer to the router – if possible – should help. But beyond that, you might want to try restarting the router and/or your computer.

If All Else Fails: In the worst case, you can plug your computer directly into your NAS via an ethernet cable. The NAS should then appear under “My Computer”, allowing you to click into it and read/write files directly. If this is faster, it shows that the issue was network related.

Hard Drives Are Naturally Slow

Do you remember when you first upgraded your computer to an SSD (from a slow hard drive)? The upgrade probably made an immediate difference. Your OS will have booted quicker, and all menus and programs seemed to load instantly. While SSDs have been around for over a decade, it’s still common to see hard drives sold en masse.

The reason for this is obvious: price. I can get a Western Digital 1TB hard drive for under $40, while the same SSD would be around $80 – twice the price. And because many of the files on a NAS aren’t accessed frequently (if you’re like me, your NAS is full of digital clutter from a decade ago!), it’s tempting to buy a hard drive and get more storage.

But there is a downside: your file transfer speeds will be much slower. A hard drive is usually 5-15x slower than an SSD. Therefore if you have become used to the fast file transfer speeds of an SSD, but then use a Synology NAS containing hard drives, you might be surprised to only get 70-80 Mbps max speeds – even though this is a normal HD transfer speed.

I actually use large SATA 3 hard drives in my current NAS, and even though I store large (1-10 GB) video files on it for video editing, I am fairly happy with the transfer speeds (because I don’t regularly need to write these files to my NAS – I mainly use the NAS for backing them up). But if sub-100 Mbps speeds sound wrong to you, you will need to ensure that you scrap hard drives everywhere.

In other words, use SSDs (NVMe or SATA) on your NAS and your computer. This will result in file transfer speeds of hundreds of megabits per second (or even over 1 Gbps, if you’re lucky).

Are You Using The Right Ethernet Cable?

I have a big box full of random junk. Well, that’s what my wife calls it. I call it really useful cables, power adapters and other essential tech peripherals.

However while there are ten or more Ethernet cables in this box, the majority are CAT5 cables. And these are all limited to 100 Mbps data speeds. CAT5e and CAT6 have faster speeds (of 1,000 Mbps), but CAT5 is quite limited in today’s day and age.

Therefore if you have plugged your Synology NAS into your home network with a CAT5 cable, you will not get more than 100 Mbps file transfer speeds. The same is true of your computer, if this is connected to your router (or a network switch) with CAT5.

The best bet is to have a bonfire and burn all your CAT5 cables. If a company ever supplies you another one with some random item, burn that too (the CAT5 cable, not the random item). Actually burning them might not be great for the environment, so instead just chuck them out.

I keep planning on doing this, because accidentally using a CAT5 cable anywhere within my home network might result in rubbish transfer or internet speeds.

Wi-Fi Is Unreliable

We all know that Wi-Fi can be buggy and unreliable. There can be loads of causes of this – being too far from the router, local Wi-Fi band congestion, or your router having too many connected Wi-Fi devices (a growing problem due to smart home technology).

While most Synology NAS devices connect to your home network via ethernet cables, you can plug in USB dongles and potentially connect your NAS to your Wi-Fi network that way (I say potentially because Synology have deprecated this approach as of DSM 7).

Equally the computer you are using (to transfer files to/from your NAS) might also be connected to Wi-Fi, opening up Wi-Fi problems there too.

To fix this, you should run through all the usual Wi-Fi debugging steps on your NAS and/or computer (whichever is connected via Wi-Fi). Modern routers offer much more reliable Wi-Fi than older routers too, so upgrading your router might help.

QuickConnect And DSFile Are Naturally Slower

Synology offer a neat feature called QuickConnect, which allows you to access your admin panel and NAS files over the internet. You literally enable it from the admin panel, and you then get a public username that you can use to access your NAS from anywhere in the world.

You can also download the DSFile smartphone app, and access NAS files from your phone (even if it’s connected to LTE or another Wi-Fi network):

It’s also pretty useful, but both DSFile and QuickConnect have a (minor) flaw: they are slower than accessing your NAS directly. That might not be surprising to many people, but it is worth discussing.

While port forwarding technology means that these programs aren’t quite as slow as they used to be, they still require all internet data (i.e. the files you are trying to access on your NAS) to pass through extra Synology services. This will slow things down a bit.

In other words, if you get 200 Mbps read speeds from your NAS at home, you will almost certainly get much less than this when you’re away-from-home and using QuickConnect and DSFile (despite this ‘flaw’, both services are still pretty useful in my opinion).

Lots Of Tiny Files Can be BAD

If your NAS has loads of tiny files, this might be causing slow NAS performance in general. This is because your Synology NAS has to maintain a cache/index of its files, in order for certain operations (like file searching) to be quick.

However having loads of really small files can make this caching/indexing process run a lot slower, causing your file transfers to be slower too. It’s a bit like having an anti-virus scan running on a (low power) computer: the whole computer is slow and laggy until the scan finishes.

To verify if this is the cause, double check the Resource monitor and see whether “CPU” or “Memory” is too high.

If everything is fairly quiet here (like in the picture above), it’s unlikely that you are experiencing this issue.

However if this is the cause, it might be a difficult one to fix depending on why you have lots of tiny files. If it’s required for a database application (for example), then disabling opportunistic locking might help.

If you simply have various folders (full of tiny files) that you aren’t actively using, you might want to consider zipping those folders up – and deleting the original folder. This will prevent your NAS from having to worry about all those tiny files.

Verify You Have Enough Free Volume Space

Whenever your disk space usage goes above 90%, your Synology NAS will struggle. This is because certain temporary files might be required during the various background tasks that your NAS performs. So it’s always good to have some extra free space as an overhead.

You can verify your disk usage by clicking the applications icon (top left on the admin panel dashboard), and selecting “Storage Manager”:

If your usage is above 90%, you should try and resolve this by:

• Deleting any unused files, or applications within Synology that you no longer require.
• Buy a bigger drive, so that you can store all your required files without deleting any. The Synology knowledge-base outlines the steps you should follow to upgrade your drives.

Check Your Drive Health

A bad drive is like a bad egg: it will stink everything up. Only in this case, the stink is… slow transfer speeds. Okay that metaphor sucks – moving on.

If any of your drives is starting to fail, this will inevitability be causing the slow transfer speeds. The Storage Manager will clearly show you if your drives are unhealthy (along with emailing you if it you have a bad drive), but this scan only runs once a month – so you should consider triggering the scan manually too.

To do this, click “HDD/SSD” within Storage Manager, then select one of your drives (left click it). Click “Health Info” and switch to the “S.M.A.R.T” tab. Here you can trigger a quick or extended test:

Tip: The extended test is naturally the most detailed, but it can be quite slow (taking many hours on a large drive). During this time, your transfer speeds will ironically be even worse! Unless you really suspect that your drive is failing, I would just run a quick test to start with. If this flags up a possible issue, I would then either run the extended test – or simply order a new drive.

Are Any Scans Or Disk Operations Running?

Your Synology NAS will periodically run certain operations, such as monthly S.M.A.R.T health checks (discussed in the section above) or rebuilding the file search cache index. During this time, performance of your NAS – and hence any file transfers – might be lower.

You should look in both Storage Manager and Resource Monitor and see whether any active operations or scans are flagged up:

Sometimes it’s not completely clear if/when a scan is running, but if your CPU or RAM usage is quite high (regularly above 50%) that might be a sign in itself that a scan or similar operation is taking place.

You might want to wait an hour and check CPU/RAM usage again. If it’s lower again, the scan/operation has completed and you should check file transfer speeds again. If they are still slow, you can at least rule out temporary scans as a potential cause.

If you have lots of applications installed (and especially virtual machines like Docker), you might genuinely be nearing your CPU or RAM limits. While you can’t upgrade the CPU, you can often add new RAM to your Synology NAS.

Monitor Drive Utilization When Transferring Files

The Synology Resource Monitor contains some really useful information, including allowing you to see the exact utilization and IOPS (input-output per second) data for each drive. To see this, launch Resource Monitor (you can do this by double clicking the name on the dashboard).

Then go to “Performance”, click “Disk” and “View Details”:

After opening this up, transfer one or more files from your NAS to your device/computer, and check to see whether any of the individual drives has a much higher utilization rate.

If one drive sticks out here (as having much more utilization than the rest), it might be a bad drive. Usually the drive health check will pick this up, but maybe it was missed by that check somehow.

Either way, high utlization can be a sign that the drive is bad – it essentially has to work much harder to access the relevant files, compared to the other drives. Swapping out this drive might make sense.

Double Check Traffic Control Settings

Synology allows you to set-up traffic control measures, where you can prioritize or slow down traffic to a particular client or application:

This is a really nice feature… when used correctly! It allows you to (for example) specify that your study PC has priority over other computers (in reading and writing files to/from the NAS). But this also has a downside: everything else will have slower speeds (bandwidth), too.

It is easy to forget a rule here, and then forget about it – wondering why another computer in your home has slower file transfer speeds.

Double check the settings in the “Control Panel” by clicking “Network” then “Traffic Control”. If you are happy with the settings here, then great. But remove any entries here that look wrong or unexpected, and then try transferring your files again.

Related Reading: Synology NAS: Turn It Off At Night, Or Run It 24/7 (All The Time)?