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 correct in that any AV will take up resources, and performance can vary depending on what shields you enable, the types of scans you run and how often, the specs of your machine, and so on. (And of course, which AV you are using -- see the results above I linked to) The key is to get something that won't slow you down and is a strong balance of protection and efficiency.