Telling someone to switch off all plugins and a theme that isn't even activated is not my idea of helping.
Then perhaps you should try being more open minded to honest helpful suggestions provided for free by other volunteers? *Drinks more coffee* ;)
FrisoP something is causing your backend to be slow. We're not mind readers here (well most of us anyways) and Andrew's advice is just basic troubleshooting. These are temporary steps just to help you identify where and what the problem is.
You may already have a backup strategy in place (after all your site is valuable) but consider this below advice.
If you want to make sure that you can get back to where you are right now and have a safety net (safety nets are good) then please consider backing up your files and database before making any temporary changes.
If you make those complete backups, store them somewhere safe (your PC or Mac is good), and you know how to restore the files and database then no matter what you do now you can put it right back via the backups.
Even if you do not use the backup it's always a good idea to have one. It can really get you out of a jam and save you some grief.