large .c files

When I open a large .c file, my pretty font-lock doesn't paint my file. The file has to be about 700 or 800 KB to do this. Of course an obvious fix is not to have .c files so big but sometimes it's unavoidable. Let me know what other info is needed to be able to get to the bottom of this. I seem to remember in the past setting in my .emacs to paint on-the-fly rather than all at once upon opening. I don't remember the setting off-hand, but it didn't work for me. If someone knows the variable that affects this, I should probably try it again on this new version of Xemacs. Thanks.