Is it feasible to render the output of css.php to a static file, or would that fall under doing over XenForo's CSS system? I ask because I checked the source and it's called on every page load and potentially doing so dynamically.
If I had to guess, it's concatenating a bunch of pre-minified CSS files and sending them to output, and this takes kind of a while. The response does seem to be cached on the server - if I repeat a request it's complete in 100-400ms but the first time (based on futzing with the k, s, l, and d params - they appear optional?) it's anywhere from 400-2400ms on my connection. The response has a valid Cache-Control header, but if you're concerned with first load rendering the result to a static file might be a good way to shave off a few milliseconds. Is XenForo spinning up a new instance every time it runs this file?