For the most part the default settings provide a good balance between performance and protection. The avast providers are set at Normal sensitivity, this slider can be increased to High for, the Internet Mail and the Web Shield providers without any undue impact on performance. The Standard Shield, I would recommend you leave on Normal, certainly for the time being. The Outlook/Exchange will be disabled unless you are using MS Outlook (not express) or MS Exchange
The other providers Instant Messaging, P2P, Network Shield providers, the slider has no effect and I believe is more to do with everything looking the same.
I would suggest that you browse the forums, especially the sticky topics at the top of each of the forums, not to mention the avast help file. They provide a wealth of information to help you get the best from avast.