At first sight I would say it is a metaphor, but after some thought I'm not sure anymore. The parallel is not so exact between the two objects, since the speed of a computer usually refers to the response speed(or more technically, measured in terms of frequency), while the speed of a rocket usually refers to the traveling speed. A more conventional metaphor is like "The boy runs as fast as a rocket.", so what is "The computer runs as fast as a rocket."?
I don't think the fact that you're not talking about literal speed matters. In fact, you could say that the use of the term speed in the context of computers is a metaphor itself - but this dooes not affect the comparison with the rocket.
Regarding the comparison with the rocket: both your examples - the computer runs as fast as a rocket and the boy runs as fast as a rocket are the same, but they are not metaphors. They are in fact similes, due to the use of the comparative phrase as fast as. Had you said the computer is a rocket, that would have been a metaphor, though arguably not a very good one!