We have unusually problem.
We used db and app server on same bad machine ( 1 CPU 400 MHZ,256 MB RAM,9 GB HDD).
We choose to put db and app server on different machines.DB server is MSCS,every node is much much stronger then before.App server is on same machine as before.
We noticed very slow work in application.
Where to start?

I guess you should start by determining what is it that has slowed down: tha app. server, the database, or the network in between.

Usually you try to eliminate possible causes.

1) Connect locally - is it slow? If so, you've just eliminated the network and app server and can focus on the database. Use snapshots and system tools to diagnose.

If not:

2) Connect from the app server box, but from the DB2 command line. Is it slow? If so, it's likely not the app server, and you can focus on network problems.

If not, then it's something the app server is doing... could still be a db2 server problem that the app server is triggering, but at least you can focus on what's going on there.

That's a good start. There's a lot more you can do for each situation of course.