That depends which one part you are actually animating.
With adding to skin.ini "AnimationFramerate: <Number of frames per second>" - This option will affect every animation that you have in your skin (except hitbursts and sliderball).
But now how to control speed with it and how to make different speed on different parts?
This is a tricky part. As stated AnimationFramrate is affecting amount of frames used per second.
Example: We have animated lifebar that contains 8 frames and animated skip button that contains 20 frames. We are setting AnimationFramerate to 4.
Full loop of lifebar animation will take 2 seconds , 5 second for skip button.
In other words The more frames you have and lower Animation Framerate is, then slower animation will be.
But now case where we want fast animation for skip and very slow for lifebar. Let's say we are using the same amount of frames as previosly stated (8 and 20)
You are setting AnimationFramerate to 10. Now you have pretty fast skip button but way too fast lifebar (less than 1 second to do loop) but you want it to be slow. So the only way is to increase amount of frames. You will need to duplicate frames few times.
Example :
scorebarcolour-0, scorebarcolour-1, scorebarcolour-2 and scorebarcolour-3 are using picture number 1. scorebarcolour-4, scorebarcolour-5, scorebarcolour-6, scorebarcolour-7 are using picture number 2 etc.
After that you will have "8 frames" as 40. So now with AnimationFramerate it will take 4 second instead less than 1.
It's not taking that much time as it's looks to duplicate frames. Generally only renaming them is a little annoying but still is possible to do this in few minutes.
(menu-back is not affected by normal duplication of frames. You need to add AnimationFramerate to control it's speed).
