A friend called me last winter, half convinced his brand new Synology was defective. He was copying a 4GB video file to it and the little progress bar was crawling along at what looked like 8 megabytes per second. On a drive rated for far more than that.
The NAS was fine. His network was strangling it.
That is the thing about slow shares: the box itself is almost never the villain. Nine times out of ten the bottleneck lives somewhere between the drive and your laptop, in a cable, a switch port, or a Wi-Fi signal that dies at the far end of the hall. This walkthrough is the exact order I check things when a transfer feels wrong, from the fastest wins to the fiddly stuff.
First, know what "fast" actually means
Before you fix anything, you need a target. Otherwise you are chasing a number you invented.
A standard home network runs on gigabit Ethernet, which tops out around 1000 megabits per second. After protocol overhead, real-world file copies land somewhere between 100 and 118 megabytes per second. That is megabytes, capital B. Your file manager reports MB/s, but network gear is sold in megabits, and the eight-to-one difference between them trips up almost everyone.
So if Windows shows a copy running at 112 MB/s, congratulations, you are already saturating a gigabit link. Nothing to fix. If it shows 11 MB/s, something is capping you at roughly a tenth of that, and that number is a huge clue on its own.
Step 1: Rule out Wi-Fi immediately
If the machine you are copying from is on Wi-Fi, stop looking anywhere else first. Wireless is the single most common cause of slow NAS transfers, and it is the easiest to prove.
Grab a laptop, plug an Ethernet cable straight from it into the same switch or router the NAS uses, and run the same copy. If the speed jumps, your Wi-Fi was the ceiling all along. Older 802.11n gear realistically delivers 5 to 15 MB/s of actual file throughput. Even a decent Wi-Fi 5 connection often lands at 30 to 60 MB/s once you are a room or two away from the access point.
None of that is broken. It is just physics. Radio waves lose to walls, distance, and your neighbour's overlapping channels every single time.
When you cannot use a cable
If wired simply is not an option for that device, move closer to the router, switch to the 5GHz band instead of 2.4GHz, and make sure nothing is streaming in the background. But manage expectations. Wi-Fi will rarely match a wire, and for large or frequent transfers a cable is the only reliable answer.
Step 2: Check the negotiated link speed
Here is the mistake I see constantly. People assume every port on their network is running at gigabit. Plenty are not.
A single bent pin, a cheap cable, or an old 10/100 switch will quietly drop a link to 100 megabits, which caps your copies at around 11 MB/s no matter what. That exact number, 11 or 12 MB/s, is the fingerprint of a link stuck at 100 Mbps.
On Windows, open the network adapter status and look at the reported speed. On a Synology, check Control Panel, Network, Network Interface, where it shows the link as 100 Mbps or 1000 Mbps. On a QNAP the same detail lives under Network and Virtual Switch.
Step 3: Suspect the cable, then prove it
Cables fail in the dumbest, most invisible ways. A cat sits on one, a desk leg crushes another, someone yanks a third out by the wire instead of the clip, and now a pair inside is broken. The plug still clicks in. The link still lights up. But it falls back to 100 Mbps.
Use Cat5e or Cat6 for gigabit. Plain Cat5, the stuff from a decade ago, may not carry a stable gigabit link over any real distance.
The fastest test is to swap the cable for a known good one and watch whether the link speed changes. I keep two short cables I trust just for this. If a fresh cable takes you from 100 to 1000 Mbps, you found it, and it cost you nothing but two minutes.
Step 4: Confirm the drives are not the limit
Occasionally the network is genuinely fine and the drives cannot keep up. This is rare on modern hardware but worth ruling out.
| Setup | Realistic write speed |
|---|---|
| Single HDD, no RAID | 90-160 MB/s |
| RAID array, healthy | 150 MB/s and up |
| Array rebuilding or scrubbing | 20-50 MB/s |
| Nearly full drive (over 90 percent) | noticeably slower |
If your array is mid-rebuild or running a scheduled data scrub, transfers will drag until it finishes, and that is completely normal. Check the storage manager for any active maintenance task before you blame the network. A drive packed to 95 percent full also slows down, so free up some room if you are cutting it close.
Step 5: Test raw network speed on its own
To separate the network from the disks entirely, use a tool that measures pure throughput with no files involved. iPerf3 is the standard, and most NAS platforms can run it through the package center or a container.
Run iPerf between your computer and the NAS. If it reports around 940 Mbps, your network is delivering full gigabit and any slow copy is a disk or software issue. If iPerf itself shows 90 Mbps, the network is your problem and no amount of drive tuning will help.
This one test saves hours. It tells you which side of the fence to keep digging on.
Step 6: Leave jumbo frames alone (usually)
Somewhere online, someone told you jumbo frames would fix everything. For most home setups, they will not, and they cause more trouble than they solve.
Jumbo frames let a network card send bigger chunks of data per packet. The catch is that every device in the path must have them enabled and set to the same size, or performance tanks and shares start dropping. One default switch in the middle and the whole thing misbehaves.
Turn jumbo frames on only after you have confirmed a clean gigabit link and you are still not satisfied. Synology's own guidance on the feature is worth a read before you touch it. See the Synology jumbo frame article for the supported MTU values.
Step 7: Rule out the software layer
If the wires and the drives all check out, look at what is running the copy. Antivirus software that scans every file in real time can gut transfer speeds, so try pausing it briefly and testing again. An outdated network driver on your PC can do the same.
SMB version matters too. Very old devices force the ancient SMB1 protocol, which is both slow and a security hole. On a modern setup you want SMB3, and both ends usually negotiate it automatically. If something on your network still demands SMB1, replace or reconfigure it rather than dragging the whole network back to it.
Putting it back together
Work the list in order and you will land on the cause almost every time. Wire the sending device, verify each link is at 1000 Mbps, swap any suspect cable, check for an array rebuild, then run iPerf to split network from disk. That sequence has solved this for me on Synology, QNAP, and homebuilt TrueNAS boxes alike.
My friend from the opening? A 100 Mbps port on a forgotten powerline adapter buried behind his TV. We wired him straight to the switch and his next copy flew at 114 MB/s. Same NAS, same file, fourteen times faster, and nothing wrong with the hardware at all.
Frequently asked questions
Why is my NAS transfer stuck at exactly 11 MB/s?
That speed is the practical limit of a 100 Mbps link, so something in your network path negotiated at 100 instead of 1000. The usual culprits are a damaged Ethernet cable, a bent pin in a plug, or an older 10/100 switch or port. Check the reported link speed on both the NAS and the computer, then swap the cable for a known good Cat5e or Cat6 one.
Should I enable jumbo frames to speed up my NAS?
For most home users, no. Jumbo frames only help if every switch, router, and network card in the path supports them and is set to the identical MTU size. One device that does not agree can slow things down or drop shares entirely, so enable them only after confirming a clean gigabit link.
Is Wi-Fi or Ethernet better for NAS transfers?
Ethernet wins for anything large or frequent. A wired gigabit connection delivers a steady 100 MB/s or more, while Wi-Fi realistically lands anywhere from 5 to 60 MB/s depending on the standard, distance, and interference. Use a cable for backups and big media files, and save Wi-Fi for casual streaming or small transfers.
How can I tell if my drives or my network is the bottleneck?
Run iPerf3 between your computer and the NAS to measure pure network throughput with no files involved. If it reports around 940 Mbps, your network is fine and the slowdown is a disk or software issue. If iPerf itself is slow, the network is the problem and drive tuning will not help.
Why does copying many small files feel so much slower than one big file?
Every file carries its own handshake and overhead, so thousands of tiny files spend most of their time on setup rather than actual data transfer. A single large file streams continuously and hits full speed. This is normal behavior, so always benchmark with one large file to get an accurate reading.
