Number of rows in Assay is roughly 300-400,
Profiles about 10.

Good point about assay and natural, however:

looking at the bottom of the SP, notice that the SP calls
another SP "ProPXGetReferenceValue" to lookup some data for
each fetched row. I have no idea how much performance is spent
in that part and the plan does not report this activity.
(While writing: an option would be to run the query with and
without the call to the 2nd procedure and compare the times...)

The problem really is that I am unsure about where to start
looking. Knowing it from application development in C++, I can
run a profiler with the program that reports me which function
is called how often, how many time is spend in it, in it with
children, etc. (e.g. Numega TrueTime).
This would be a nice thing to have for databases - specifically
Firebird ... dream :-)