Performance drops down when using MSAA and with setting camera.setRenderTarget = null

Hi Everyone,
I accidentally find a strange thing when I keep MSAA*4 Open and set the camera's render target to null.
It drops down the fps from 30 to 20, and GPU Usage from 80% to 100%.
I detected this problem from two android mobiles.
I can't figure it out, so anyone willing to help me?
Thank you so much!