Yea, Kaspersky antivirus does have better detection rates than MSE (especially when it comes to detect currently "unknown" malware, using heuristics engine) or majority of other paid and free antivirus programs, but it is also notorious for random performance issues (and is very "sensitive" to the "leftovers" of previous antivirus programs). The random slowdowns don't happen for majority of users, though, and they do try to eliminate these issues with each new release. It's also highly customizeable - for example, in latest version there's "game mode" which, when enabled, can supress some warnings and some actions if you are currently playing some game, or another setting where you can tell the antivirus to start the auto-update only when your PC is idling. Plus it has a 30-day trial mode during which you can fully test the antivirus to see if it can cause performance issues on YOUR particular system.
Edit: here are the screenshots I made from latest version of Kaspersky Antivirus - with all those options enabled, you should have no significant "slow-downs" of your PC, like with previous versions of this antivirus:
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