1]
Things can change every second.
That is why they need to be scanned each time.

2]
Perform a clean installation of the latest avast version and see if things improve.
FireFox, Chrome and Internet Explorer are by default slow browsers.
If you want to try a much faster browser:
ftp://ftp.opera.com/pub/opera/win/1217/int/

3]
Facebook is sending a lot of unneeded things/code.
Nothing avast can do about.