I am having a similar issue, though not exactly the same. When I open a large PHP file (~3700) lines scrolling or any editing becomes completely useless. Scrolling (using keys in vintage mode) causes 1 core to max our for 3-4 secs, then scrolling occurs. Smaller PHP files seem to be ok.
This is happening on my new laptop, Core i7 2720QM, Fedora 15. Opening the same file on a windows machine (AMD X2 64 4200, 2GB RAM) and it is a tiny bit sluggish, but nothing like my new system.
Seems like the linux version needs some tweaking maybe?