It doesn't decrease system performance noticeably (actually not at all on my systems) when the shields are set to high. Standard shield level is important for me actually. With normal level, it doesn't scan mp3 files for example. And this is a bad case for those who use peer to peer applications. I have some friends who gave up using avast! because of missing malicious mp3 files (which are detected by manual scan), even though i told them it was because of the level of the standard shield (p2p and im shields work with standard shield). And this lack of scanning some extensions isn't limited with mp3 files.
It's better to have an antivirus program that blocks malware in the first place.
This is up to user, though (depending on his/her usage purposes and system resources). Let her/him use with both normal and high settings, if s/he notices a performance impact on her/his system, then s/he can stick with normal level. But i don't think unless s/he has a old granny computer.
Btw, s/he asked for the best settings, which means "the highest settings" for most of the users who seem like novice asking so many basic questions.
