C-n is very slow in Font-Lock mode

[Richard Stallman]

With Font-Lock mode enabled, starting from the beginning of a large
file (such as fileio.c), C-n with a large argument (I tried C-u 2536
C-n) took many seconds.  I think it was fontifying all the text
that it moved across.

Can this be avoided?  I don't think C-n ought to need to fontify
all that text.

Even once it has fontified, the command still takes a lot longer
with Font-Lock mode enabled than it does with Font-Lock disabled.