Manual:$wgRunJobsAsync

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

Details
When running jobs asynchronously, it will open an internal HTTP connection for handling the execution of jobs, and 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 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 load of the page.

Note that even if $wgRunJobsAsync is set to true, if PHP can't open a socket to make the internal HTTP request, it will fallback to the synchronous job execution.