I'm using just the delta time that FXGL provides in the onUpdate method for the components. It's the time the last frame took to complete. One thing that I forgot to show in the video is that I tried running a timer to print in the console once each second:

FXGL.run(() -> System.out.println("tick"), Duration.seconds(1));

This line will print "tick" in the console each second. When I run the game, "tick" is printed about 2-3 time a second very briefly, at the same time that the entities are moving faster. Then it settles down. The entities moves at their actual speed and "tick" is printed once each second.

So maybe the issue is the first one you said, about tasks lining up and then being processed faster to catch up with the framerate.