Q. I've often heard that WRKACTJOB takes up a lot of CPU power.  Why?

A. Over the years there have been changes to the way WRKACTJOB works. What was
   once true for DSPACTJOB on System/38 is no longer true for WRKACTJOB.
   Unfortunately people still pass on the old recommendations. In the
   following paragraphs I will attempt to provide an up to date description of
   WRKACTJOB.

   When you exit WRKACTJOB by using Enter, F3, or F12 you have totally ended
   WRKACTJOB. The statistics reported by WRKACTJOB are not collected by
   WRKACTJOB. The collection of those statistics is always done for every job
   in the system by LIC. This collection occurs even when WRKACTJOB has never
   been used on the system.

   WRKACTJOB does keep an internal space somewhere to save the current
   statistics so that it can report the changes the next time it is used
   within the same job. You can reset those statistics by using F13=Reset
   statistics or by using the RESET(*YES) parameter on the command. Using
   System Request followed by option 2 will cause WRKACTJOB to destroy the
   internal space it uses to save its statistics. It will then create another
   space the next time it is run. The space is also destroyed at the end of
   the job, so there is no advantage to using System Request followed by
   option 2 to end WRKACTJOB. The only WRKACTJOB resource you release when you
   sign off is the internal space object used to store the statistics. The
   size of this space varies based on the number of active jobs in the system.
   By signing off and then on again after each use of WRKACTJOB you are
   forcing it to recreate that space in the new job. This will waste more
   resources than it saves. Since WRKUSRJOB does not report any job statistics
   it does not need to use a space to save them.

   In the past, the reason for the recommendation to not use WRKACTJOB (or
   DSPACTJOB on System/38) was because of the resources that it used while it
   was building the data to be displayed. In the past WRKACTJOB searched all
   work control blocks on the system looking for active jobs. When the system
   had lots of jobs with a status of JOBQ or OUTQ it could take WRKACTJOB a
   relatively long time to find just the active jobs. This was changed several
   releases ago. Now WRKACTJOB uses a much faster LIC interface to materialize
   the list of active jobs.

Answer courtesy Ed Fishel via Midrange-L