GIF playback should be efficient but...

About twenty years ago I was generating long animated GIFs. They worked fine in Firefox. In Internet Explorer they started fine but became jankier as playback progressed. I realised that every time IE displayed a frame it was rereading the entire file from the beginning to get to the current frame. Which took longer and longer as the current frame advanced.

It's just so easy to squander performance without noticing.