Large downloads cap around 1 MB/s on a fast line
A legacy firewall or NIC driver can break TCP window scaling. The connection peaks and drops without ever hitting line speed.
Try this first
- 1Test bandwidth with iperf3 over LAN and then over WAN
- 2Capture in Wireshark and check whether the Window Scale option is in SYN
- 3Update NIC drivers and disable advanced offloading temporarily
- 4Do not disable Receive Window Auto-Tuning, that usually makes it worse
When to bring us in
For an in-path firewall: have the network team check SACK, ECN and window-scale stripping.
See also
- My laptop is suddenly slowThree main suspects: a runaway background process, near-full disk, or a Windows update in progress.
- One specific application is slow for everyoneIf an app is slow for one person: local. For everyone at once: server side or vendor side.
- Opening files from the file server takes minutesOften the server itself is fine and the issue is network routing or stalling DNS resolution.
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