If your antivirus actively watches every frikin' thing you do, it's going to slow down your computer to a crawl. This is the approach a few of them take, and it's annoying. you can turn off most of those resource-heavy "imma watch you" features, but then it's cheaper to get something free. Prevention + anti-malware + passive antivirus will work fine.
Prevention is mainly plugins for your browser:
ublock origin (chrome, firefox)
WOT (chrome, firefox)
ghostery (chrome, firefox)
sandboxie (for testing/running shady things)
anti-malware:
ccleaner (free)
Malware-antibytes (free)
passive antivirus:
what other people say in this thread. I use windows defender.
Another good bet is to make sure to get rid of all the windows10 bloat. They made it for laptops+everything, so remove all the +everything bits and your laptop will thank you. I've removed cortana and many of the features I will never use but take resources.