I doubt that the skins themselves are the main issue here.
If you are getting non acceptable fps in osu during gameplay, skins are not the problem here.
Some skins have playfield borders which can reduce fps, as well as having bad spinners and other things like that, but as Zelzatter Zero said the default skin is probably the best option.
Next best thing is getting a computer upgrade, although I can understand why some people can't/don't want to upgrade their computer.