I do not understand yet David's perspective on speed not affecting kihon. In all the keiko I have done, I have found that speed does affect how I perform at the edge of my competence and ability. I think that success at using aikido under pressure often means moving in a relaxed and somewhat unussual way, and doing that quickly under pressure (for me) is more difficult than doing it slowly. I see that hold true for many of the people I have trained with (in my eyes in any case).
Speed is a very interesting issue... Much of the issue of speed has to do with perception rather than some hard reality. When you perceive the attacker's actions as "fast" you feel as if you need to move faster yourself. If you are "in reaction" to the partner / attacker, this produces a perception that one doesn't have sufficient time to execute ones technique and this causes mental and physical tension which then tends to degrade performance.
We do an exercise which we call "time shifting". You take a person who is having trouble during a three person randori, usually because they are too excited to execute technique properly. The nage is instructed to do the randori slowly, almost in slow motion. he is asked to chant to himself while doing the randori "I have all the time in the world. No reason to get excited..." He should do this the whole time he is doing the randori. The ukes are told to go full speed, which would seem to be a contradiction...
We have been doing this during our randori intensives for over 15 years. In 100% of the cases performance increased rather than decreased. the nage was far better able to execute the movement required, they stayed calmer, and their sense of time "shifted". They stopped feeling as if they had to out speed the ukes. instead, they saw things much slower. This allows them to make better movement decisions and be more precise, therefore successful, in the techniques they did.
Eventually, you can access this way of looking at things by developing a relaxed and calm emotional state. You see things as happening at a far slower speed than you once did. It's very interesting. That's why the training needs to be structured to develop this proper attitude and perception. if the training isn't constructed properly yo0u simply imprint an excited state of mind and this type of perception never develops.