Webpages that crash
Well, that might be partly due to the connection speed, but mostly it is the mobification and scriptification of the websites. A website that pulls in scripts from two dozen sources to just display the frontpage of whatever is slow and annoying (plus the layout is now for mobile devices, even if there is an app, and to see anything interesting you have to scroll past the full width banner with some f'ing slideshow of stupid irrelevant pictures, which again eats resources and data like crazy, and then there are more full width and full pc-screen height banners down there).
Sorry - yes, I'm annoyed, why do you ask? But yes, connection speed is of course the main culprit *shakes head*