I’ve had a few others over the years (Doepfer A-101-2, Optomix, Sputnik quad, LxD), and I’d say the Synthrotek I built with the default vactrols is on the longer decay side.
It sounds good and generally works well, but I don’t understand why designers put CV attenuation on the control inputs–these should be offsets! Optomix is the same way, so it’s not a specific knock on the Synthrotek.
The resonance is on the wild side and not as useful to me as the Doepfer’s very flexible resonance & input level controls.
Overall, I like it a lot better than the LxD it replaced. Beware of the depth!