Compared with my previous use of Firefox, browsing with Tor runs very slowly. Is this because Tor Browser is loading ads that you can't se because of ad blockers?
Well, this case can be fixed a bit - if you have a good connection. You need to add
IsolateDestAddr to your SocksPort directive in config. But be warned - if the website uses multiple (sub)domains - in some rare cases it can misbehave. The method here is to use different circuits for different destination addresses. So ads will be loaded via separate circuit and the parallelism of the whole process will be a bit better. But circuits are circuits - you're not insured from having a slow circuit in any place.