The whole point of PoW is that it requires a given amount of effort at a given level of difficulty to maintain a given level of production. (i.e. it's a function of the capital cost of the equipment and the operational cost of running it) All else being equal, if the amount of effort to produce a given result is reduced it will result in an increased level of difficulty netting out to a similar level of energy use.
This is the reason that Bitcoin, for example, keeps ratcheting up the difficulty: to counteract the increased performance of CPUs, then GPUs, then FPGAs and finally ASICs over time. It's an arms race that you can't 'win' for any extended period of time since the difficulty is not a constant, but rather determined by the desired level of production.
This is something that I appreciate about ethash (Ethereum's current PoW algo)... it is tied to GPUs (and memory controllers). One side effect is that older GPUs tend to drive higher ROI numbers vs. an arms race of always having the latest smaller nm ASIC technology.
Oh and CPUs don't work well because that drives people to create botnets. FPGAs kind of sit out there in an esoteric expensive and difficult to run island all by themselves.
This is the reason that Bitcoin, for example, keeps ratcheting up the difficulty: to counteract the increased performance of CPUs, then GPUs, then FPGAs and finally ASICs over time. It's an arms race that you can't 'win' for any extended period of time since the difficulty is not a constant, but rather determined by the desired level of production.