Slow is subjective. People make weird mistakes about speed all the time. Anecdote: My dad used to move the mouse in circles (the pointer tracing many circles around his web browser window) because he 'knew' it speeded up his Internet connection.
Trimming those lost cycles causes some problems - code might be harder to read and understand; you might be introducing bugs; you might be competing with the compiler; etc etc. That's all fine if the gain is worth it, but for most people it really isn't.
It would be nice if machine start up could be a bit speedier - people seem to have forgotten about the 5 second boot of the ASUS EEE PC. I guess this is a good demonstration of the problem you talk about - vendors could tweak and recompile the linux kernel and tweak settings, but they don't.
[1] For some values of 'some' including 'most'.