Theproblem with (Realtime) virusscanners is that they're inherently heavy on the CPU, as they have to hash and compare every file you open to a antivirus database. In my experience Avira is one of the lighter scanners, you could try exclude large folders that you deem always safe, but that's very risky on an OS that doesn't have proper privilege separation and allows anyone to write into any directory.
Most of the antivirus programs are demanding a lot of resources, from my experience. You could try Avast antivirus as a free solution. Generally speaking ESET NOD32 proved to be a reliable antivirus with not so high resource requirements.
The best antivirus is the person that uses the PC: if you're not accessing odd websites, download torrents with cracks, hacks and other similar programs, you'd be just fine with the Malwarebytes scan from time to time
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