Variable: jit-lock-stealth-nice

Time in seconds to pause between chunks of stealth fontification.
Each iteration of stealth fontification is separated by this amount of time,
thus reducing the demand that stealth fontification makes on the system.
If nil, means stealth fontification is never paused.
To reduce machine load during stealth fontification, at the cost of stealth
taking longer to fontify, you could increase the value of this variable.
See also `jit-lock-stealth-load'.