Is there any antivirus software that DOESN'T slow down Time Machine?
I'm not really sure if this fits the ask different format, however, according to AV-TEST (the closest thing to an authority on the subject) BitDefender comes out very high on their scores, especially on the 'slowdown' tests.
I do use it on Windows &amp; it's a lot less irritating than some I could mention. 

Avast on the Mac is a known resource hog, btw. I could even feel it on a dual 6-core Mac Pro.
oops, Avast/Avira... they're both near the bottom of the speed tables, so I'll leave it as is