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.
I believe in Kaspersky more than any other anti virus. Its named after its owner Eugene Kaspersky. Kaspersky may be little bit heavier than Avast but would be capable of delivering better results.
If you look at the content generated by the kaspersky’s team you will find that they are successful in identifying many potential threats before actually it affects the computers to a large extent. There are other anti virus products such as mcafee, norton, quickheal and bitdefender which are considered to be good anti virus products
The one that I felt is the best is avg anti virus. It's light and it's somewhat free. But nowadays, virus attacks seem to be less relevant as i guess hackers don't really target small time pc users nowadays. More effort I guess is being diverted into building technologies that are involved in block chain technology.
I have used other anti virus and some are really heavy on the system and lags the whole pc into use frustration. The other thing you can use to improve the pc security is to install malwarebytes. Quite good to detect Mal wares. Once in a while it is really important to get your pc scanned. There was this time that I experienced such an extreme drop in my Mac books battery life that I later discovered was the work of tons of virus that were on the computer.
I downloaded avg and updated the database and scanned the system. Removed all those unwanted parasitic virus and used malwarebytes to scan for Mal wares. After clearing out all of those stuff, my Mac books battery life improved and was back.
Do scan any pendrive that you put into the pc and maybe get a browser that extends the virus capabilities. Most virus enter into the computer from browsers. Usually if any deadly virus comes into your pc and something has been affected. Just Google it to find the solutions.
Usually there are plenty of solutions out there. Nowadays virus doesn't really harm out computers anymore.