1. Take any sufficiently big PHP file (slightly exaggerated example file).
2. Open it in php-mode.
3. Copy any line or block of code (even an empty line).
4. Paste it. It's going to be noticeably slow.
   If running on a supercomputer, spam that paste chord a bit (the pastes will queue, it will take a while).
   Undoing the operations will be similarly slow.

The slowness is increased the bigger the file.

It seems like the culprit is