Best AV for Older, Slower Computers?

Can't seem to find anything useful on this...and I thought I was a
pretty decent researcher.

I want an effective antivirus app, but it should be lean and fast so
that an older system isn't slowed to a crawl.

Low cost/free is nice, too.

Surely someone is sucessfully targeting this niche?

Thanks for any leads,

Nat

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