nowadays Kaspersky become so much famous until it went bloated with many unnecessary (not really needed) programs. these memory/cpu processing hogging eventually slowing down your system. -- now that's what we call unfair comment

anyway, normally any Anti Virus will use cpu power to scan/real time scanning and depends on what AV as well how much CPU raw power got. even with 8-core also lagging still got.

FYI, Microsoft have this Microsoft Security Essentials, it's free, and it's lightweight, scanning also very fast as well as very good in virus/malware detection. you should try.