It's a rather mundane topic, but important to your clients when the server is overloaded: FileMaker Server 15 Top Call Logging:
FileMaker Server 15 has the ability to track the heaviest client usage periods. Top Call Statistics Log gives admins the ability to pinpoint the client requests
And if that is not enough, Soliant Computing has a video, too:
And don't forget Nick Lightbody and Rick Carlton's version (previously posted), as well:
FileMaker Server 15 Top Call Logging is one of those major, hidden features that have a large impact but little exposure outside of the developer world. But if you have bad acting server, you may want to upgrade to FileMaker Server 15 sooner than later just for this feature.