The single frame that takes the longest time to render won't normally be an issue, but a cluster of frames each taking significantly long than usual will result

The single frame that takes the longest time to render won't normally be an issue, but a cluster of frames each taking significantly long than usual will result in a noticable stutter. The stats should try to take into account not only the low frame-rate but whether that rate has been held long enough to peturb a human observer.