Anti-Virus Resources Speed

I just switched from McAfee to Sophos and everything seems to be moving much faster, much less drain.

And it got me wondering if there is a lot of differences between the anti-virus programs, not in ability, but in speed, size, drain, usability.

From everything I've read, the top ten anti-virus programs aren't that far apart in ability to protect so I was wondering what you guys think are the best as far as size, resources, usabilty.

If this has already been discussed, I aplogize.