I think it's confusing because it makes it seem like you are setting the fps for the project, which you are not by all means! You are only setting what the timeline markers and stepping references, secondly what if you want to have one animation at 12 fps the other at 24 in one project?
The bar in Spine allows more incremental changes, it just snaps to the common resolution if you get close to them, like 24.08 it would snap to 24. In Construct it could just be a value field in the timeline Properties.
I agree the speed thing is kinda redundant.
And the Stepped, Interpolated is not easing. We already have a similar dropdown in Construct Timeline though (called Animation mode).
Imo if you rename duration you would need to rename it to hold for duration or freeze for duration. As otherwise it would not imply correctly what you are supposed to set there.
But I don't want to argue, I think it's completely fine to have different opinions on how to solve certain things, I just want to give my pov on this. So I hope you don't see this as an attack. :)