Yes, an SD project would run faster and more efficiently than HD but...
...the speed, power and screen size of the hardware would define how noticeable the difference is to the user. If you are running on a small portable device with very little ram and a slow processor, you would probably want to use lower definition images. If your game is only intended to run on full PC systems with fast processors and lots of ram, the speed difference would be minimal and the user experience would be better at a higher definition.