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Usually managers will be questioning on the amount of time a job runs. Previously job would have executed in less time, but with days the same job with same input might take more time to complete. This would begin discussion of performance improvement. But before even looking into the code, we should first see the CPU time used for the job on different days, if it is same, then the question is to analyze other factors that are mentioned below.

CPU time is the amount of time the task is actually executed for

Elapsed time represents the total amount of time the task has been around
(Elapsed Time = Waiting time of the Job + CPU Time )

Under z/OS, there are typically hundreds of tasks running simultaneously on any given system. These include started tasks (which would include DB2, MQ series, Websphere, system tasks), TSO users, batch jobs, CICS regions. The system manages these tasks and allows each to run for a certain amount of time each minute.

Some of the Factors that may constitute to elapsed time for a batch job:
CPU utilization
Number of jobs in system
System overhead
Initiators in use
Tape contention
Disk contention
Channel contention
Job priority
WLM policies

The exact same job can use the exact same about of CPU time and the elapsed time may vary by hours — due to the above-mentioned factors, among others.