Compared to i3wm on Linux, it takes ages. I can could probably switch workspaces three or more times (using the keyboard) during the time that a single Spaces animation takes to complete.

Also, I am on High Sierra too, and there is at least a half-second lag between when the gesture ends and when the animation is complete. Taken together with the time it takes to initiate the gesture, I'd say we're around 0.75s.