Re: C-n is very slow in Font-Lock mode

[Richard Stallman]

      JIT lock is optimized for C-v and similar methods that page
    by large chunks of text rather than by one line.

I am not sure what that means, in concrete terms of Lisp program
actions.  I'm also not sure what you mean by "make text visible".

It turns out that vertical-motion was fontifying all the text that
it moved over.  That seems to be easy to fix.