This is more of a question than a comment, but here it goes: what do y'all think about Steve Pavlina's blog ( http://www.stevepavlina.com/blog/ ) , when it comes to the issues raised by the OP?
I'm not sure speed alone, by itself, is a solution. If you speed down a game by, say, 70% there would probably be no difference than if you sped it down by 90%, since there's a limit to what the character can do in a given second. Mario, for instance, once you jump, there's not much to do until he actually lands.
Suppose the same would happen if we had the capability to speed down time in our actual lives. Sure you could dodge bullets and win F1 races from time to time, but the actual day-to-day tasks, that take the majority of our time wouldn't be improved much. If you need to eat lunch, eating it in an optimal way won't give you much advantage in comparison to regular people that don't take the fork to their mouths following a perfect parabola.
It is said by some that intelligence is the capability to adapt, to change, while rationality is doing the most logical thing. For instance: a computer that makes investments by itself - a rational computer, using logic to maximize profits. A computer that invents something - a smart, intelligent computer, that has developed some level of intelligence and is using that to chang its environment instead of merely working on it.
Never thought of it this way. Guess in the long term it makes sense. So far, though...
My guess is the second greatest good would be the best option. Taking into account that these are mortal beings and you don't know how long will these people live/be healthy, the best choice would be one that is not the least rewarding (first greatest good) neither the slowest to come.
Think of it as spending your life savings: you don't want to spend it all on your youth and be poor the rest of your life and you don't want to spend it all in your nineties either, when you'll be too old to enjoy it. The answer is somewhere in the middle.
Playing devil's advocate here, the original poster is not that wrong. Ask any other living species on Earth and they will say their life would be better without humans around.
Since you won't be able to kill all humans and will eventually get caught and imprisoned, the best move is to abandon your plan, accordingo to utilitarian logic.