To make Font Lock mode faster for large buffers, you can use Lazy Lock mode to reduce the amount of text that is fontified. In Lazy Lock mode, buffer fontification is demand-driven; it happens to portions of the buffer that are about to be displayed. And fontification of your changes is deferred; it happens only when Emacs has been idle for a certain short period of time.
The command M-x lazy-lock-mode turns Lazy Lock mode on or off, according to the argument (with no argument, it toggles). You can also arrange to enable Lazy Lock mode whenever you use Font Lock mode, like this:
(setq font-lock-support-mode 'lazy-lock-mode)
It is not worth avoiding buffer fontification for small buffers.
Therefore, the variable lazy-lock-minimum-size specifies a
minimum buffer size for demand-driven buffer fontification. Buffers
smaller than that are fontified all at once, as in plain Font Lock mode.
When you alter the buffer, Lazy Lock mode defers fontification of the
text you changed. The variable lazy-lock-defer-time specifies
how many seconds Emacs must be idle before it starts fontifying your
changes. If the value is 0, then changes are fontified immediately, as
in plain Font Lock mode.
Lazy Lock mode normally fontifies newly visible portions of the buffer
before they are first displayed. However, if the value of
lazy-lock-defer-on-scrolling is non-nil, newly visible
text is fontified only when Emacs is idle for
lazy-lock-defer-time seconds.
In some modes, including C mode and Emacs Lisp mode, changes in one
line's contents can alter the context for subsequent lines, and thus
change how they ought to be fontified. Ordinarily, you must type
M-g M-g to refontify the subsequent lines. However, if you set
the variable lazy-lock-defer-contextually to non-nil, Lazy
Lock mode does this automatically, after lazy-lock-defer-time
seconds.
When Emacs is idle for a long time, Lazy Lock fontifies additional portions of the buffer, not yet displayed, in case you will display them later. This is called stealth fontification.
The variable lazy-lock-stealth-time specifies how many seconds
Emacs has to be idle before stealth fontification starts. A value of
nil means no stealth fontification. The variables
lazy-lock-stealth-lines and lazy-lock-stealth-verbose
specify the granularity and verbosity of stealth fontification.
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