I believe the TCPIP stack at os390 2.4 could only use 1 CPU.  When you added
a CPU to your machine the actual MIPS for each individual CPU goes down.
They call it the MP effect.  This may be playing a part in what you are
experiencing.  But there really are many many things to look at.  As you
added an engine and memory did you look to see if TSM and or TCP is actually
using more or less memory/CPU resources?
Matt