Timing related updates are in, now Skull 'n' Bones can be played back with the correct speed, even with VSync toggled on (and a refresh rate lower than 70Hz)
Looks like VSync got mistakenly enabled by default for Catacomb 3-D at some point (even though it's OK now, following the timing updates). It further was the case for Keen Dreams with CGA graphics.
CatAbyss: Objects folder dependency missing from Makefile (thanks Blzut3)