For the site imgur.com it depends on the link posted as to what we do with it. If you post a default imgur sharing URL (they changed this recently) then it’ll work differently in different browsers, it runs a script to see what you are on and then picks something compatible. A top level link would look like this (i.e. no file suffix anymore):
On desktop that will be a mp4 video playing as a stream, on mobile (if it can’t support it) will default back to a .gif but only load when clicked. So then it’ll look like this:
For .gif files we automatically download and store a copy locally here (if not too big), so if the original gets deleted the topic isn’t showing a bunch of broken images/gifs (RE: photobucket saga). Unfortunately imgur.com is starting to ‘downgrade’ the produced raw .gif file, which means the mp4 (and using the root sharing URL) is nearly always better.
Imgur.com is trying to stop people post direct .gif links but people still do. It’s basically so they can insert ads like Youtube at some point in the future.
As for topics being slow on scrolling, the forum only loads what it needs as you scroll (we don’t have pages, which is kind of weird) so you’re nearly always good doing whatever. We also do a ‘image too large’ click thing to speed up big images (like direct gifs) as well.