I'm on v1.6.5 and there was no problem before. I have to clear topic cache, so I can fix the solved questions images. When I do that, the forum don't respond, just stucks. But it still works, the button does what it must. After 1-2 minutes, I open a new tab and everything's normal then.
Today my forum has 14,9 posts and 5400 topics. I guess I have that problem after 5k topic or around like that.
We know about the issue. Please don't worry about this, it is already under development.
In the next version release, all the bulk refreshes will become partly refreshable and the issue will be fixed.