To a first approximation, TZ is just an extra bit on addresses. So, probably not so much.
Well, decryption has costs - and the question was DRM and not TZ specific. But yes, doing the work in the secure world is as cheap as the normal OS
There's no decryption. It's encrypted all the way to your screen's each pixel.
That makes no sense. You might not be able to get at the decrypted pathway from software, but at some point there's got to be a decrypted linear buffer to drive the pixel array.