I use Kaspersky, i havnt really noticed any difference in speed since i installed it, when i first downloaded it it kept screaming at me "literally" cos i had viruses, but once that had died down it was quite good, i havnt really touched it since then exept to update my licence.
I havnt tried it, i might on my next computer though, Shield Deluxe, was voted the best antivirus program on a number of websites, you might want to check that out as well.