font-lock-support-mode

font-lock-support-mode is a variable defined in `font-lock.el'.
Its value is

jit-lock-mode

Documentation:
Support mode for Font Lock mode.
Support modes speed up Font Lock mode by being choosy about when fontification
occurs. The default support mode, Just-in-time Lock mode (symbol
`jit-lock-mode'), is recommended.

Other, older support modes are Fast Lock mode (symbol `fast-lock-mode') and
Lazy Lock mode (symbol `lazy-lock-mode'). See those modes for more info.
However, they are no longer recommended, as Just-in-time Lock mode is better.

If nil, means support for Font Lock mode is never performed.
If a symbol, use that support mode.
If a list, each element should be of the form (MAJOR-MODE . SUPPORT-MODE),
where MAJOR-MODE is a symbol or t (meaning the default). For example:
((c-mode . fast-lock-mode) (c++-mode . fast-lock-mode) (t . lazy-lock-mode))
means that Fast Lock mode is used to support Font Lock mode for buffers in C or
C++ modes, and Lazy Lock mode is used to support Font Lock mode otherwise.

The value of this variable is used when Font Lock mode is turned on.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 21.1 of Emacs.