I’d say it’s not a drain, but even as a long-time ad-blocker, I still am conditioned to ignore things screaming for attention. It’s been very often that I’m looking for something (eg. a link) but I don’t see it because it’s more visible than the rest of the page and my brain is conditioned to filter that out.
It's a constant cognitive drain on your mental resources. That's even ignoring the fact that almost everyone is influenced by ads to varying degrees.