Manual:$wgJobBackoffThrottling

Details
Number of work items of a given job time to perform per second. What exactly a "work item" means is up to the job; it just has to return the number of them in. For example, it could mean the number of pages updated for a job which does batch updates.

Whenever a job of the given type gets executed, no jobs of the same type will be started in the next ( work item count / backoff throttling) seconds.

This only affects jobs started by maintenance/runJobs.php (as opposed to $wgJobRunRate which only affects job started by web requests).