Maybe I can try to reduce Texture size on CC & Rebirth, it can help the framerate...
Already one step ahead of you there, but I'd suggest turning texture filtering off in the code additionally.
I've created 2 skins for Opsu: 1 is the default one aliased, so without any alpha channel, removing the need of pretty slow alpha blending and backgrounds scaled to 800x480, the other one is the same but 50% size on textures that get scaled appropriately in-game.
I recommend the first 1 for gigahertz users who want a few frames more, the second one for rebirth and CC users in general, as it makes the game actual playable with stable 20 fps.
Of course as rebirth/CC user, you should also turn off all effects, turn off background dimming and maybe also only use the default background.
I've tried to make them look a bit better than just your simple batched aliasing and downscale, so it actual looks acceptable on the Pandora.
Oh, those go into "appdata/opsu/Skins" btw.
opsu_aliased.zip