To answer @avivey's question more directly, it takes a long time for the yellow Loading... bars to expand to the diff for files I'm interested in because the server is spending all it's CPU time generating diffs of generated files that appear first on the page.

It's true that they're loaded in parallel using AJAX but there are enough generated files that it's tying up the server's CPU. And the generated files seem to end up first in the queue.