recently this blog post

  http://tsengf.blogspot.de/2012/11/slow-scrolling-speed-in-emacs.html

suggested to set `jit-lock-defer-time' to some small fraction to defer
jit-lock fontification a bit.  This has an enormous effect for example
when scrolling in large c-mode buffers.

However, most of the time I work with buffers that are rather small and
instant fontification is fast enough.  There, deferring font-lock is
somewhat distracting.

So is there a way to enable deferred jit-lock on a per-buffer basis?

I tried this:

  (defun th-jit-lock-defer-fontification ()
    (interactive)
    (set (make-local-variable 'jit-lock-defer-time) 0.1)
    (font-lock-mode -1)
    (font-lock-mode 1))

In fact, when I execute that command, the variable is buffer-local, and
jit-lock is deferred as it should.  But it is actually deferred in *all*
buffers!  Even disabling and re-enabling font-lock-mode in a buffer that
doesn't have a buffer-local value of `jit-lock-defer-time' doesn't make
fontification instant again...

Any ideas?

--8<---------------cut here---------------start------------->8---
(defun th-jit-lock-defer-fontification ()
  (interactive)
  (make-local-variable 'jit-lock-stealth-timer)
  (make-local-variable 'jit-lock-stealth-repeat-timer)
  (make-local-variable 'jit-lock-context-timer)
  (make-local-variable 'jit-lock-defer-timer)
  (set (make-local-variable 'jit-lock-defer-time) 0.1)
  (font-lock-mode -1)
  (font-lock-mode 1))
--8<---------------cut here---------------end--------------->8---

and it seems to work exactly as expected. :-)