Manual:$wgRunJobsAsync

When the execution of jobs during normal page requests is enabled (by setting   to a number greater than 0; it defaults to 1), then this variable controls whether to execute them asynchronously or not.

Details
When running jobs asynchronously, an internal HTTP connection for handling the execution of jobs will be opened, and MediaWiki will return the contents of the page immediately to the client without waiting for the job to complete.

Otherwise, the job will be executed in the same process as the returning of the page and the client will have to wait until the job is completed.

When the job does not run asynchronously, if a fatal error occurs during job execution, it will propagate to the client, aborting the loading of the page.

Even if   is set to true, if PHP can't open a socket to make the internal HTTP request, it will fall back to the synchronous job execution.