We're also fighting this issue for quite a while. This seems to be related to macOS and Rclone only.
The way that macOS handles processes / threads works differently that on other OS's and despite our efforts to isolate Rclone jobs, so they use spare "work threads" instead of main rendering thread.
It still sometimes happen as if macOS decide that the main thread isn't busy, so "let's not waste the CPU resources and assign heavy task to it" - that makes the app UI unusable.
Fix is likely couple lines / config changes long, only if we knew what to tweak.
May I have your machine spec and OS version please?
Reasonable start from our end, would likely be to provide configurable number of concurrent threads/transfers, so user could tweak that number and see if it helps on their end.
We're really keen to have this resolved! (edited)