On a track with 128 frames, 256 rows per frame, 4 effect columns for all 28 channels, when playing at the beginning of the last frame, this feature took around 0.45 second on average. In real scenarios there is usually a slight delay of a 60 Hz tick for every ~7000 rows. Most of the time the tracker has to traverse through all rows to confirm that the 7xy effect is never used.
This could be vastly improved if every pattern keeps a cache which is updated upon loading a FTM or editing; that approach also makes it possible to retrieve the state of the Fxx effect (and in 0CC-FamiTracker the groove position) from all previous frames without having to traverse all the pattern data twice.
Post about feature requests here.