A lot of the ad JS I see is in the form of inline script tags, which generally should not block anything (the JS usually asynchronously constructs another script tag, which shouldn't impact performance).

>Try it, the change can be dramatic.

I've been using AdBlock Plus, and now uBlock, for at least 8 years. So I'm definitely not arguing against it.

It's just that in theory, an ad tracker (like a 1 pixel image) does not necessarily have to impact performance. Also note that some ad blockers add performance overhead themselves.