I too have been using windows defender for the past 5 years and have had no issues till date.
And antivirus does slow down your system.
After reading your reply I did google.
My answer did sound naive. I would like to change my reply but am no longer able to edit my reply.
Since i don't use antivirus. I shouldn't have replied to this thread in the first place.
Anyway, these are the changes i would like to mention :
As
@HoNeYBiRD mentioned. The first layer of security is the user. No matter how good the antivirus is, you can get compromised.
It's you as the user who needs to be careful. Avoid installing cracked software.
There is an organization named av-comparatives. They are an independent organization which tests various antivirus software.
Google them as i don't know if i can add there link here.
NOD32 is regarded as to be the better one in regards to the performance.
But coming to malware blocking Kaspersky is the better alternative.
Windows defender sucks in offline mode.