button.mouseHover change button background image slow

I have written some code to change a button's background image on mouse-hover, however when the image is changes it seems that it is a bit slow to complete.

That is, the开发者_如何学编程 process of setting the new picture as its background image is slow. By slow I mean a few milliseconds, but still enough to be seen!

Is there anything I can do about this, to speed up this process?

let use mouse move (the mouse overed control)