Using latest version of Lightroom on a very high spec PC, 32GB ram, i9700k, GTX 2070 graphics card etc., all drivers up to date. Issue also happens on simlarly high (but different) spec laptop.
Didn't used to have this issue with Lightroom CC, but when syncing, all other connection to internet freezes. Usually, I find file manager also freezes - e.g. cannot open folders on the PC. Issue goes away only on PC reset. It's a pretty decentish fibre connection (40MBs download, 8 upload).
The issue is ONLY when Sync is turned on in Lightroom. All works fine & fast when sync is off, and also fine when already synced. BUT I can only sync by putting computer on, then just leaving it for hours and not opening any other apps or programs, otherwise get the freezes.
As this happens on both my computers, seems to be a genuine issue.