Avast vs. Norton - Avast seems to slow down OS

I've noticed a definitely delay since I loaded Avast 4.5 with all it's
shields. I know that the Standard Shield and mail shields are necessary,
but is the Outlook/Exhchange, P2P, and Network Shields necessary if you're
not on a network and just using a cable modem (although some say that's
like being on a MONSTER sized network).

What's everyone's opinion of the Script Blocking? Necessary or not?

I know I see a slight delay every time I hit select an item on a web page.