Performance in NIST

Hello,
we have tried to make some tests to check performance. We're using a Sun 
Solaris machine with 4 cpu's. The scenario is:

SIPP                          NIST
INVITE ------------------------->
<----------------------------- 200
ACK ----------------------------->
BYE ----------------------------->
<------------------------------200

We see the stack processes less than 30 cps,
but the CPU usage is not very high.

We made another test, this time acting as a B2BUA:

SIPP                          NIST                      SIPP
INVITE ------------------------->
                                 INVITE -------------------->
                                  <-------------------------200
<----------------------------- 200
ACK ----------------------------->
                                 ACK ------------------------>
BYE ----------------------------->
                                 BYE ------------------------->
                                  <-------------------------200
<------------------------------200

This time the stack supports 40 cps (20 in each end) with also a not very 
high CPU usage.

It seems that perhaps there's something blocking (a bottle neck?) the input 
of the messages. We've tried to use several listening points and different 
configurations.

Regards,
Fran.