Suddenly slow sync Android to Linux (via USB tether or Wifi)

I use syncthing to sync most of my Android phone onto my laptop. A while ago I noticed that Wifi is kinda slow in general, but I can quick back up GBs of data if I tether my laptop to my phone using USB, with download speeds of (I think) tens of MiB/s.

But today, when trying to do that, the download comes to a crawling halt. A few hundert KiB/s if I am lucky.

I am really wondering what broke. I might have upgraded syncthing a minor release or two since the last time I ran it, now at 1.10.0, and upgraded Android from version 10 to version 11. Nothing odd in either logs.

Could the latter be a cause? Is Android somehow trying to throttle the app?

It seems that going over Wifi is also slow (slower than I think it used to be).

Hmm, it sped up after a while, although not reliably. Maybe the app was still indexing and syncing and indexing at the same time is too slow…