Extremely slow local uploads (KB/s)

NC 14.0.4 on DS218+ (Celeron, 2GB, dual 2TB)
LAN access to disk is up to 30-40MB/s upload.
Completely default install

Nextcloud client 2.5.1 on macOS has upload 2-5KB/s (e.g. hours for 100MB).

Where should I even begin to look for the problem?

Do you have a lot of disk activity e.g. with other programs/services running (file sync tools - anything that’s reading large directories)? If so, and especially if it’s a physical drive (not SSD) then it could simply be due to disk access time swamping your system.

I did that once using nextcloud’s command line sync tool - I had several running at once (because I’m stupid) and my transfer speed went to KB/sec rates.