Had this problem with cmucl src/code and src/compiler directories a while ago but it was fixed.
However, try getting the file list for ansi-tests/ansi-tests and scroll down a bit. It brings firefox to its knees. Chrome is a bit better but it still takes a while on my 3GHz imac.
Compare this with github.com/rtoy/ansi-cl-tests. It's very smooth and takes no time at all.
(It would probably be nice for its own sake if ansi-tests put related tests into subdirectories instead of all the files in one directory. But that's a different issue.)
-- Ray