Because of all the JavaScript. I swear like 90% of the Web nowadays is just JavaScript. Look into the NoScript add-on. It allows you selectively block (or allow) JS scripts on any given webpage/site/etc.
It actually makes it SO much faster to load webpages. Plus the privacy & security benefits are pretty nice too. 👍 :)
I, and most people in fact, only use a handful of sites these days. And most of the sites you only really need to block the most common tracking scripts and the site totally works fine and loads noticeably faster.
And you only have to do it once, not every time. It might take a bit of time the first go around but then every time after that it’s out-of-sight-out-of-mind most of the time in my experience.