Job not not getting it's fair share of CPU??

I've got about 5 users, each running the same screens over the same
data. One set of users is getting decent performance, while the other
set of users is not.

Job run priority, time slice, class, ect are the same.

Some days a particular user was one of the ones getting good
performance, other days not.

Talking to the user's manager, one comment stood out...the users
getting good performance (that day) had gotten an early start.

I seem to vaguely recall reading/hearing something about work
management issues that could be caused by large number of "identical"
jobs running in the same subsystem and that the lower numbered
(named?) jobs would tend to get the resources. The users getting good
performance where getting twice (10-20%) of the CPU vs. the users
getting bad performance (5-10%)

The issue started after our full backup and IPL on the weekend of the
19th. Since the screens having the issue made use of SQL for
searching my guess was that a very useful system maintained temporary
index was deleted at IPL. Actually, I think I've figured out the
missing index and have since created it. So CPU usage is down and I'm
waiting to hear from the users. But I'm curious as to why some users
saw good performance and others didn't.

If my memory is correct regarding the problem of "identical" jobs,
that could explain the difference in the users job performance for the
users running this one particular set of screens given that all 700+
of our interactive users run in one subsystem using the same priority,
time slice, ect.

Can anybody confirm my recollection or otherwise shed some light on this?

Thanks!
Charles Wilt