As far as
AV goes I use Comodo CIS.
http://www.comodo.com/home/internet-security/security-software.phpI don't have any problems with leeching being slowed down.
I took CIS off recently as I had a Kaspersky Internet Security license given me - Kaspersky was uninstalled within 24hrs & I was back with CIS which I find much more configurable & controllable.
I don't like being told what I can access & that it's not up to me to decide what is hostile.
CIS once setup gives me a secure FW environment (on it's custom ruleset) & doesn't auto delete files that I am trying to edit & use! & not to sound too much like a fanboy but the defense+ has stopped a few nasty malware installers that have appeared of late.
Over the last 10 years I have had Bitdefender, Norton (was & still is one of the worst ever IMO), AVG, Panda, (was quite good a while back), Bullguard (rivals Norton for uselessness) & McAfee.
Every time I give these others a shot, I just end up going back to CIS, it works, it gives me some control & it's free.
As a case in point I had a new laptop with the usual free trial & it's nag screens insisting I buy the license or my world would end. I figured I'd see how it got on for the 30 days, even tho it really annoyed me at times. Then I uninstalled it & put CIS on.
Updated it & scanned & the list of malware missed by the paid solution appeared.
1 virus 'proper' & more than 30 malware components had got in. (stuff like trackers, url hijacks etc)
So as a 'power' user I say use CIS, it doesn't seem to create any lag (comparatively) & gives you control, if you know nothing/don't need to know about setting stuff up or how malware works, free CIS still works but the paid for Comodo with online live assist or Kaspersky might be better bets right now.
The pre-installed free trials on new machines may as well be uninstalled right after you power up the machine in my experience.
Another related piece of freeware is
hashtab, which is a cool way of checking files for integrity, tampering etc, useful in these days of infected downloads
http://www.implbits.com/hashtab.aspx