NAS slow transfer speeds over 1GbE network

You're copying a 10 GB video file from your Windows 11 PC to a Synology DS220+ (or QNAP TS-453D) over a wired 1GbE network. The transfer starts okay, then drops to 30-50 MB/s. Sometimes it jumps to 100 MB/s for a second, then drops again. You've checked cables—they're Cat6. Router is a Netgear Nighthawk AC1900 with all ports gigabit. So what's wrong?

What's actually happening here

Your network hardware can do 1 Gbps, which gives you about 110-120 MB/s in real life (after overhead). But two things kill that speed:

• SMB version mismatch — Windows 10/11 defaults to SMB 3.1.1, but some NAS models (especially older firmware) negotiate down to SMB 1 or 2. That adds latency and limits throughput.
• Jumbo frames mismatch — Your NAS might have jumbo frames enabled (9000 MTU), but your PC or router doesn't. Packets get fragmented, which actually makes things slower.

The fix — step by step

1. Check your current SMB version on the NAS
   On Synology: Control Panel → File Services → SMB → Advanced Settings. Look for "Min SMB protocol" and "Max SMB protocol". Set both to SMB3. On QNAP: Control Panel → Network & File Services → Win/Mac/NFS → Microsoft Networking. Set SMB version to SMB 3.0.
   Why step 1 works: SMB 3.0+ supports multi-channel and encryption offload. Older versions don't, so your CPU gets hammered.
2. Disable jumbo frames on everything
   Go to your NAS network settings. Look for MTU. Set it to 1500 (standard). Also check your PC's NIC driver settings (Control Panel → Network adapter → Properties → Configure → Advanced → Jumbo Packet → Disabled). Router doesn't matter for MTU if it's not fragmenting.
   Why step 2 works: Jumbo frames only help when every device in the chain supports them at the same size. Most consumer routers don't. You end up with packet reassembly overhead that slows you down.
3. Check your ethernet flow control settings
   On your PC's NIC driver settings (Advanced tab), set:

Flow Control: Disabled
Large Send Offload (IPv4): Disabled
Large Send Offload (IPv6): Disabled

Reboot both NAS and PC after changing these.

4. Test with a direct cable (PC to NAS)
   Unplug the router. Connect PC and NAS with a single Cat6 cable. Assign static IPs: PC 192.168.1.10, NAS 192.168.1.20 (subnet 255.255.255.0). Copy a large file. If speed jumps to 110 MB/s, the router is the bottleneck (bad switch chip or QoS). If still slow, the issue is in the NAS or PC NIC drivers.
5. Update NIC drivers on your PC
   Realtek and Intel both have had buggy drivers for 1GbE. Go to the actual chip vendor's site (Realtek.com or Intel.com). Do not use Windows Update drivers — they're often outdated. Install latest stable.
6. Change SMB settings on Windows
   Open PowerShell as admin, run:

Set-SmbClientConfiguration -EnableBandwidthThrottling 0
Set-SmbClientConfiguration -EnableLargeMtu 1

Then reboot. This tells Windows to use bigger packets (up to 1 MB) over SMB, which reduces CPU overhead. Why it works: By default Windows limits SMB packet size to 64 KB. That's fine for Wi-Fi but bad for wired.

If it still fails

• Run iperf3 — On your PC and NAS, install iperf3. Run iperf3 -s on one, iperf3 -c [IP] on the other. You should see 940-980 Mbps. If not, your cable or NIC is bad.
• Check NAS CPU usage during transfer — If CPU is pegged at 100%, your NAS is too weak for encryption or you have many services running (Docker, Plex transcoding). Turn off file encryption on SMB and disable any antivirus/scanning on the NAS.
• Switch to SMB 2.1 — Rare, but some old NAS firmware handles SMB3 badly. Set Min and Max to SMB2 and test.
• Try NFS instead — On Linux/macOS clients, NFS often gives better speed than SMB because it's simpler. On Windows, enable Services for NFS (optional feature).

Most people with NAS slow speeds have either wrong SMB version or jumbo frames enabled on one device only. Steps 1 and 2 fix 80% of cases. The direct cable test (step 4) tells you exactly where the bottleneck is. Don't buy a new router until you've done that.