Manual:$wgJobBackoffThrottling

Details
Number of work items of a given job type 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. However, this setting won't limit the number of pages updated in one job run (which is controlled by ). Instead, it will throttle new executions of that job type.

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  (as opposed to  which only affects job started by web requests).