This issue may occur if the Job Count exceeds the recommended limits.
The maximum number of running jobs that an SCVMM system should be able to handle is around 10,000 and hitting this many running jobs at one time would be an exceptionally rare occurrence. Should it occur, however, it can have a severe impact on the performance of the SCVMM engine.
The number of retained Jobs count refers to the number of running tasks tracked in the TaskTrail database table in SQL. The size of this database can have as much of an impact on performance as the actual running jobs. If the number of running jobs nowhere reaches 10,000 then it may be that the tombstone limit on entries tracked in the TaskTrail database is too conservative
By default the TaskGC entry in HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft System Center Virtual Machine Manager Server\Settings\SQL is set to tombstone entries at 90 day intervals. If a user is running a number of jobs on a daily basis that would cause the TaskTrail database to exceed this tolerance then this number can be reduced to tombstone entries more often.
By default, a job runs every 20 hours that will read this value and start the process to remove the jobs above the upper recommended limit, however it may take several cycles to bring the total job count down to acceptable limits.