Yesterday, I mentioned that Xah Lee had a short post on dramatically differing run times for processing several files with
find-file versus processing the same files using
with-temp-buffer. His post was fairly short and didn't have a lot of details. Happily, he's published an extended post that shows the code and detailed performance of the two approaches.
This is really interesting and if you write Elisp that looks at lots of files at once, you should check out his post.