I'll try, but this will take some time, very busy at the moment.
I suspect that it's either the amount of active tabs (or something that's happening within those tabs) or an extension that's playing up. So for me also discovering if it is an extension, and which one is important. So I'd rather first test with the current situation.
Is there a process monitor in Dev tools with which I can see the process that occupies the CPU at the moment when I get a beach ball. What do I look for? And how do I use it?