Manual:$wgJobRunRate

Details
Number of jobs to perform per request. May be less than one in which case jobs are performed probabilistically. If this is zero, jobs will not be done during ordinary apache requests. In this case, maintenance/runJobs.php should be run periodically.

Explanation
The job queue is designed to hold many short tasks. By default, each time a request runs, one job is taken from the job queue and executed. If the performance burden of this is too great, you can reduce $wgJobRunRate by putting something like this in your LocalSettings.php:

This will cause one item in the job queue to run on average every 100 page views. It is important to understand that this means that on every page view the probability of running a queued item is 1 in a 100. This means that (in theory at least) you could still end up with one job being run every page impression, or (at the other end of the scale) no jobs being run at all. However, in practice, providing you have enough traffic to make a meaningful sample size, it should be about 1 per 100 requests.

You can view the current length of the job queue by visiting Special:Statistics (providing you are using v1.6.0 or higher). Note that this figure is cached, so it will not always be completely accurate.

The job queue is located in Wiki.php (function doJobs).