Pasting lines is slow in large files

1. Take any sufficiently big PHP file ([slightly exaggerated example file](https://p.teknik.io/K1GuH)).
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 https://github.com/emacs-php/php-mode/blob/c53e0c1ec154fcfe6e5db56d1513d503a62e0355/php-mode.el#L954
Notice there are no heredocs at all in the file.