Thanks for the mention elbert.
OP, your AV shouldn't slow down your machine. As far as we're concerned, our team works very hard to make sure Avast won't slow down your system (and we have results to prove it too, see AV-comparatives: https

/blog.avast.com/avast-and-avg-only-free-antivirus-score-100-av-comparatives-real-world-test)
That being said, I can't say "which is best," I'll leave that up to the community, but if you have any questions about Avast I'm happy to help or make recommendations on which option might be best for you based on your needs. It might be good to know which features you need or would like, and if you're looking for a free version or something with extra functionality.
On a final note, too, elbert is...