A checkbox wouldn't do much. Framerate limiting is something that's done at runtime. People have requested that feature before though.
The thing that's misleading on the editor side are the numbers on the timeline. Spine doesn't store animations in terms of frames so there's technically no such thing as a "sub-frame". It saves keys in terms of seconds and fractions of seconds and everything else is interpolated on a continuous scale.
@Shiu. Would it be cool if the timeline had graduations/lines between numbers to suggest Spine's continuous instead of discrete-frame nature? Then people can immediately understand that there's always something between the numbers the timeline snaps to... hopefully. At least it's one more thing to hint at it.
