cryptonight is both memory and compute hard - the same as with zec.
10% is compute hard. 90% wasted waiting for memory. My cryptonight mod is running full speed @ 50% tdp. Most of the gpu is not used.
but speed is rising when you overclock gpu core. Both amd and nvidia. The same is for zec.
For ETH hashrate has upper limit bounded with memory and if you o/c gpu clock above some value there is no hashrate increase