Tracing IIS / OWA bottlenecks?

Does anyone know of a good way to hunt down bottlnecks in
going to particular IIS systems through the web? We have
an older 5.5 exchange system which users can access using
OWA over a 2000 server running IIS 5.0 native version.
The issue is that the speed at which users can access this
system is very slow so I was looking for some way to trace
down where the link is in a bottleneck. This is ussing
HTTPS: as well as going through another VPN using Secure
Client software. Now using this software to just VPN in
and use Outlook is no problem but the web mail is way to
slow and sometimes even times out. So any ideas or help
you could give would be most appreciated!