You upgraded to a 10 Gb NIC and switch, yet copies top out at 110–118 MB/s. That number means something in the path is linked at 1 Gbps (1 Gbps is roughly 115 MB/s in practice). If you are stuck at 200–500 MB/s instead, the bottleneck is usually the disk or PCIe, not the network. Work through these checks in order.
1. Check the negotiated link speed
# Windows (PowerShell)
Get-NetAdapter | Select Name, InterfaceDescription, LinkSpeed
# Linux
ethtool enp3s0 | grep -i speedIf it says 1 Gbps, the cable or switch port is the culprit. Many switches only have a couple of 10G uplink ports, so confirm the port you used is actually 10G using the spec sheet and the port LED color.
2. Cables and modules
| Type | What 10G needs |
|---|---|
| RJ45 (10GBASE-T) | Cat6a for 100 m; Cat6 to about 55 m. Cat5e is out of spec and often falls back to 1G or 2.5G |
| SFP+ DAC (copper) | Short runs of a few meters; check NIC and switch vendor compatibility lists |
| SFP+ optics | SR (multimode) or LR (single-mode) modules must match on both ends and match the fiber type |
Cat5e in the walls, or a 1G patch panel or keystone in the middle, will drag the link down. If in doubt, connect the two devices directly with a short Cat6a cable and compare.
3. Measure the network alone with iperf3
File copy speed mixes in disk speed, so it cannot tell you where the problem is. Use iperf3 to measure raw network throughput:
# On the server (NAS or another PC)
iperf3 -s
# On the client: 4 parallel streams for 10 seconds
iperf3 -c 192.168.1.10 -P 4
# And the reverse direction
iperf3 -c 192.168.1.10 -P 4 -ROver 9 Gbps means the network is fine and storage is the bottleneck. Stuck at 3–6 Gbps points to the PCIe slot, CPU or driver.
4. PCIe lanes and disk throughput
| Component | Approx. max | vs 10G (~1,150 MB/s) |
|---|---|---|
| Single HDD | 150–280 MB/s | Bottleneck |
| Single SATA SSD | ~550 MB/s | Bottleneck |
| NVMe SSD | 2,000 MB/s+ | Enough |
| PCIe 2.0 x1 slot | ~500 MB/s | Bottleneck |
| PCIe 2.0 x4 / 3.0 x2 or better | ~2,000 MB/s | Enough |
Some x16-sized slots are wired as x4 or even x1. Check the lane count in the motherboard manual, especially the second slot under the GPU. On a NAS with only a few drives, the array itself often cannot fill 10G, and that is normal.
5. Drivers and SMB settings
- Drivers: install the vendor driver (Intel, Marvell/Aquantia, Mellanox, etc.) instead of the inbox Windows driver.
- Jumbo frames: MTU 9000 (or 9014) only works if the PC, switch and NAS all match. One mismatch makes things slower or flaky. Gains are usually 5–10%, so try this last.
- SMB: set the NAS minimum protocol to SMB2 and maximum to SMB3. Synology can enable SMB3 Multichannel under advanced SMB settings.
- Real-time antivirus: it can eat CPU during large copies; check it while testing.
Cheat sheet: what your number means
| Result | Look at |
|---|---|
| Flat ~115 MB/s | 1G link: cable or switch port |
| ~280 MB/s | 2.5G link or a single HDD |
| ~550 MB/s | SATA SSD limit or a 5G link |
| iperf3 9G+, copies slow | Disks, SMB settings, antivirus |