"Slow is smooth and smooth is fast".
True. It is a very useful paradigm.
Take a pistol draw stroke. The best way to see and feel where the inefficiencies, excess tension, and wasted motion in a draw stroke are is to do them slowly. The same thing holds for aikido technique.
However, the only way to really get fast is to... go fast. This gets really obvious with something like splits or target transitions. You can't learn to see the sights when shooting sub-.2 second splits with a pistol if you don't shoot sub-.2 second splits. Obvideolink: http://www.youtube.com/watch?v=XAnnK63PqF8&NR=1
(No, that's not a machine gun. Yes, he reloaded in the middle of all that.) Jake Di Vita, the gentleman in that video, didn't get that fast by going slow and smooth. He pushs past the comfort zone and gets a little sloppy (but not so far as to be unsafe), then pulls back. Repeat ad nauseaum. The comfort zone gets inexorably pushed farther and farther out this way.
Also, if you spend all your time going slow and smooth, you can easily run afoul Mroz' Law: Anything at all will work at 1/2 speed and 1/2 force, Most things will work at 3/4 speed and 3/4 force, A surprising number of things will work at 7/8 speed and 7/8 force, but almost nothing works at full speed and full force.
Just because it works slow and smooth doesn't mean it will work when things get interesting. I think alot of martial arts drift into this sort of problem if they don't have an "alive" component in them.