Avira is antivirus software, whereas Windows Defender isn't (it defends against other kinds of malware), so you need both.
However, I would try an alternative AV to see if you can find one which uses less system resources.
I eventually settled on Comodo, although I use their Internet Security suite rather than just the AV:
Assuming you're running Windows 10: https://antivirus.comodo.com/antivirus-for-windows-10/