Before the "buildVisitor()" method in LocalInspectionTool appeared, it was quite easy to measure the execution time of an inspection. Just add some System.currentTimeMillis() and you're done. This was a good way to spot performance problems in the inspections of InspectionGadgets.
Currently all InspectionGadgets inspections implement the buildVisitor() method and the actual running of the inspection is done in IDEA code. A way/api to measure/retrieve the run time of an inspection would be very useful.