I work with some files that have some complex fontification rules. There is a significant lag when scrolling quickly through these files. Although I am using jit-lock-mode as my font-lock-support-mode, the lag is unbearable. One solution to this is to use fast-scroll. https://github.com/ahungry/fast-scroll. fast-scroll temporarily disables font-lock when scrolling. This has worked out nicely for me.