Stylesheet Profiling is a very useful technique for tracking down processing bottlenecks in stylesheets.
Please have a look on the two slides from last year's WSTE presentation:
Unfortunately the time column did not report timing values as milliseconds (as it should) but in sub-microseconds.
The real value are not that important as for performance tuning the relative correctness of the reported times was good enough to work on transformation bottlenecks.
The incorrect timing values have been fixed in April fixpack:
IC74751: THE XSLT PROFILE TIMER COLUMN SHOWS INCORRECT VALUES WHEN VIEWED IN WEBGUI OR VIA COMMAND LINE INTERFACE (CLI).
Find additional information in this developerWorks DataPower forum posting:
"Re: Need to find the processing time of a transform action in a policy"