Turn on the 'performance adjustment' - some admins with experience on older versions of OS/400 turn this off (it used to be really bad). To turn it on, use:
wrksysval qpfradj
and select '3' (automatic adjustment).
If you don't want to turn on the performance adjustment, memory pools can be adjusted manually with wrksyssts - use f11 for different views.
I've seen the performance adjustment (post v4 OS/400, anyway) make dramatic performance gains, though...