The job or your entire system is I/O bound. The CPU is sitting idle waiting for data off from disk. Memory is under utilized due to poor caching of data. The job is likely performing millions if not billions of logical I/Os to retrieve its data for processing. Most applications do a fairly good job of adding, updating and deleting records from your
database files. They do a very poor job of reading only the necessary records for processing. Better database tuning for an SQL based application or more efficient keyed database access for your RPG or COBOL application might be just what is needed.

If your system has 5 processors and this job consumes 20% of the CPU, it is consuming 100% of 1 processor. Single threaded jobs only use 1 processor at a time.