Keeping Avast is honestly your best bet, for two major reasons;
A) If it's letting malware into your PC chances are it's actually your fault. You need to make sure the software is updated at all times, and combine it with some browser-based help. I have uBlock as an adblocker, because sites not monitoring their ads is where a lot of malware comes from so blocking those ads outright prevents a lot of that. By practicing these safe internet actions and keeping Avast up to date I've never had malware in over a decade of PC ownership.
B) Any antivirus with quality is going to slow down your computer during scanning. If it's not then chances are it's not doing a good enough job, honestly. It's just something you have to put up with to keep your computer safe. Think about it like this - would you rather schedule your scans and have your computer run slow for a little bit or risk the software not catching stuff that's in your computer because it doesn't scan deep enough and as a result doesn't slow down?
Also Avast has a free option that I've been using for as long as I've had it, which is great when you're low on money or broke or whatever and don't have to worry about paying a monthly or yearly fee for an antivirus. The free version of Avast has served me just fine, and as long as you're smart about it can do the same for you too.