> Webpages only load nearly instantly

Which ones? HN does, but a large part of the other sites I frequent need around a second or more to load. With or without ad-blockers - some load faster with, some faster without, but they never enter the "nearly instantly" range, which (last I checked) was 0-250 ms.

What's an example of a site that loads slower with an ad blocker? I've never observed that in my browsing.

I'm sure you can find examples with particularly poorly written adblockers but everybody should be using uBlock Origin and it is extremely efficient.