jit-lock-stealth-load

jit-lock-stealth-load is a variable defined in `jit-lock.el'.
Its value is

200

Documentation:
Load in percentage above which stealth fontification is suspended.
Stealth fontification pauses when the system short-term load average (as
returned by the function `load-average' if supported) goes above this level,
thus reducing the demand that stealth fontification makes on the system.
If nil, means stealth fontification is never suspended.
To reduce machine load during stealth fontification, at the cost of stealth
taking longer to fontify, you could reduce the value of this variable.
See also `jit-lock-stealth-nice'.

You can customize this variable.