Large SFTP uploads become extremely slow when nearly finished

I'm uploading folders containing a small number of large video files to a remote server.

My speedtest.net results show that my effective internet speed over WiFi is 600 Mbps down, 100 Mbps up.

My uploads using WinSCP start at a fairly decent 80 Mbps but, when they are nearly complete, the upload speed drops to around 1 Mbps. Interestingly, while WinSCP is struggling to upload files, a speedtest.net test will still give full download and upload speeds, so other applications are not experiencing slow connectivity - just WinSCP.

I've tried three things and have identified the step required to get my uploads back to normal:

- pause WinSCP upload + resume uploads in WinSCP = no improvement
- pause WinSCP upload + disconnect from WiFi/reconnect + resume upload = no improvement
- pause WinSCP upload + DISABLE WiFi NIC/re-enable WiFi NIC + resume upload = upload returns to full speed (approximately 10x faster).

Any idea why I'm seeing slow uploads towards the end of WinSCP transfers, which can only be resolved by disabling then re-enabling my NIC?

Please attach a full session log file on level Debug 1 showing the problem (using the latest version of WinSCP).

It looks like this was happening due to something else. I'm opening a new ticket.