I think I’m seeing this issue too on a Samsung Galaxy s10 after upgrading to Android 11. For me, it’s not the file transfer process that’s slow, but the scanning process.
I have a Send Only folder on my PC with ~10k files, 90 GB (MP3s) and a Receive Only folder on my phone’s SD card.
Each time SyncThing goes through the scanning process (which seems to be when starting the app and at the end of the transfer process), it takes around 20+ minutes. It takes longer if the app is in the background or if the device’s screen is off. Before updating, the process was relatively quick: probably 30 seconds or less.
In the logs, I see “Folder […] isn’t making sync progress - retrying in #m#ss”. Not sure where else to look for information.
My SyncThing app is not battery optimized either.