I agree with all that is stated above I just wanted to point out that in my experience fast is easier, and there for I think that slow with good intent is far more valuable especially in the beginning. I know that people fall into the trap of I need to do it fast so it will be realistic but I think that in the end you have to learn how to move first. after all the harder that they attack the more off balance they are, and the more energy they are giving you to work with.
that being said I still like to go fast now and again! I think variation is important, and I have seen and heard that if you really are "doing aikido" then speed doesn't matter. I think that O' sensei said something like they attack thinking I am in front of them when in fact I was behind them even before they moved. I have seen this in my sensei for him speed does not seem to matter.
"Do you thing my being faster than you has anything to do with my muscles, in this place?"