In your screen fade are you trying to CROSSFADE, or just to fade to black or white?
On my iMac the glCopyTexSubImage2D() which copies part of the backbuffer into an existing texture, seems to work at full speed, suggesting it is hardware accelerated, otherwise if it went out to main memory it would be much slower framerate. I use it for a full-screen de-res effect for which I have to copy the whole frame to a texture and then draw the texture.
That said I don't know that all systems have this hardware accelerated, some may be software which is much slower.
On my iMac the glCopyTexSubImage2D() which copies part of the backbuffer into an existing texture, seems to work at full speed, suggesting it is hardware accelerated, otherwise if it went out to main memory it would be much slower framerate. I use it for a full-screen de-res effect for which I have to copy the whole frame to a texture and then draw the texture.
That said I don't know that all systems have this hardware accelerated, some may be software which is much slower.