I have committed new code that measures script execution time. In the viewer, the "Script Time" field in the "Statistics" panel now shows a value; previously it was 0. And the "Top Scripts" window contains more accurate and actionable timings than before.
This includes information about the different kinds of "Ticks" in OpenSim, and about a general-purpose sliding window class that I added, called MetricsCollector. This class can be used for many other things besides measuring scripts performance.