Actually I just saw you have nearly 4000 tags and there's a well known performance issue for users who have many tags. So in the end I don't think it's related to sync - it's just that while sync is in progress, the tag list is being constantly updated which probably freezes the UI.
It's an issue we want to address and some work is being done for this, but not sure when it will be ready unfortunately.
I don't assume that collapsing the tag list helps? (if you click on the "TAG" header in the sidebar)