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I don’t have information for every antivirus suite out there, but Tom’s Guide tested 5 popular free choices: Avast, AVG, Avira, Malwarebytes Anti-Malware Free, and Microsoft Windows Defender. The good news is that Windows Defender had very little impact on system performance, and bested the others in that respect. The bad news is that it was the worst at actually doing its job and detecting malware. It wasn’t just a teeny, tiny bit worse, it was “no one should be relying on it for PC protections” worse.
Tom’s Guide picked Avira Free Antivirus as the best overall, but they did note that it could impact performance on low RAM PCs. AVG, which is what I use on Windows 7, was picked as the best choice for Windows 8. It was also described as having minimal impact on system performance.
Here is the link to the article if you would like to see more details.