Would it make threads load slower?
It makes things a bit cleaner here, I think, without the addition of images in signatures. They might impact page load times, yes, since signature capsules are set to a height of 0 instead of a display of 0.
Well, signatures are hidden for the most part, so it wouldn't affect anyone just reading through a thread.
They do pop up when you're giving karma, though, so I can understand the decision.
Why can't I embed an image in my signature?