What I have heard is if you have a newer computer with Windows Vista or Windows 7 and a good chunk of RAM (4GB or more), Windows Defender is an average malware blocker. It is more of a resource hog as a download for the older Windows XP. If you are noticing slowdowns with your system, as long as Avast is activated, you can disable Windows Defender if you want. If your computer's speed or RAM is not affected by it, you can leave it on and you'll be just fine.
It all depends on how it behaves with your current Anti-Virus software, But remember, Windows Defender is not a substitute for your updated anti-virus software. Before I reformatted the hard drive on my XP system, I used to have Windows Defender and other security software. It seemed to get slower as new versions came out. Now, I just use Avast and Malware Bytes on Demand, which works fine.
Jack