Have ZoneMinder-MP-1.25.0-47.1.x86_64 installed, blew away the database and recreated it.
If Monitor/Misc/Frame Skip is zero, fast forward on replay of events works as expected - video is smooth but sped up.
If Monitor/Misc/Frame Skip is nonzero (4, in my case), fast forward on replay of events has little if any effect. Video consists of a series of distinct stills with brief pauses in between. Timestamps on video show that fast forward setting is basically ignored - even at 50x I get about one still per second, and timetamps on stills show they were taken 4-10 seconds apart.
Very annoying, and worked fine on 1.22...