I've never had an ad-on slow me down noticeably but it's possible if it's a resource intensive ad-on. If you go into your task manager on your computer you can see how much memory (ie ram) each program running on your computer is using. It won't separate the ad-ons out but at least you can see if it s firefox or some other adware/spyware program running in the background that you can't see. That might help you narrow down what is causing the problems.