A bit vague because I'm only an occasional W10 user (...I use it for Lightroom/Photoshop and now't else) but when you start Task Manager you should see a simple view that shows your running foreground applications. At the bottom of the dialog is a button to 'Show All Processes'. If you click that, you should see a table with all foreground/background processes, with columns for CPU, Disk, Memory etc. If you click the column headers, you can sort the processes by their CPU/Disk usage etc. Clicking and sorting the columns should give you an idea as to what the biggest resource hogs are.
Modern MalwareBytes does install memory resident realtime virus protection, so you probably don't need this as well as Avast. You can open the MalwareBytes panel and disable the realtime protection.
Quite honestly there's not an AV product out there that I like at the moment. They all seem to have there flaws. We rolled out BitDefender at work. It was problematic with the likes of OneDrive when users had lots of files. We switched out to Panda, purely because it could be installed via Group Policy. I've seen it irrecoverably trash one machine and cause problems with another.
Preferable to just rely on Windows Defender and practice safe browsing habits. I use Firefox with the 'noscript' plug-in to restrict java script etc. and always (always) type URLs (complete with the 'https') rather than click links etc.
