[TChris]

EVERY anti-virus program will slow down your computer while it's scanning; it's impossible NOT to.

I turn off the automatic scanning, or set it to scan in the middle of the night.

[Justa]

Avast! has the lowest footprint for active scans I’ve seen. I think it runs at lower disk priority than User during scans, unlike SAV which runs higher than User. Avast! also has many configuration options and features.