I find the hardest thing with regard to practising slowly is keeping the fast parts relatively fast so that the technique doesn't end-up being monotonous in speed and still shows realism. You still have to show the fast (positive action) parts as fast, comparatively, and the slow (cautious) parts slow.
We had a very good explanation of jo, ha, kyu given by Yabe sensei via Yoji-san at the summer seminar and it wasn't at all like the basic explanation usually given. Jo, Ha, Kyu translated as slow, faster, fastest is really not at all correct. Yabe sensei explained it like a play where in Jo is the opening scene, ha being the interplay of the characters (in this case you and teki) and kyu being the conclusion.
My perception of this in nukitsuke is jo - where you have mentally decided to take action against the enemy, so it's a time of mental preparation. Ha - as you take action where you are watching and assessing the enemy as you are making your draw (who knows, he may be much stronger or faster than you and you may have to defend), and Kyu - where you make decisive action, cut nukitsuke.
Although pressure and speed do build to a climax it's much more involved than simply slow, faster, fastest.
I'm still fighting with the cutting aspect myself. There's no point putting a huge amount of energy in the very start of the cut because all you're doing is getting the sword to the target (extending the elbows, wrists), it's only when cutting the target that you need to add power. Now this is the hard bit, how do you add power without using the wrong muscles, certainly not using the shoulders, this is where timing of your tenouchi, using the arms, lateral muscles and body comes in. Unfortunately, from my experience at least, it's not something anyone can teach you and the only answer really is to keep practising with an aim to improve. John Donaldson said something that really made me think about the body, he said when you're relaxed your body talks to you and when you're tense it doesn't. With this in mind I found out in my training what he meant, if you're relaxed you can actually FEEL when things are right / wrong, your body gives you feedback, if everything is tense and tight then you don't get that feedback.
John H.G. had some valid points on our kiritsuki, basically your aim is to have the power where you need it which is as you cut the target. A good way of checking this with an iaito is from the sound. You're not looking for a sound from way back at the beginning of the cut, you're looking for a pronounced sound from the entry point of your imagined target, the kime you're searching for. The cuts don't have to be fast, just make the focus of the cut the intended target.
Many of us seem to have gotten kihon exercises merging into the waza because no clear definition, to my mind, has been made between the two. Drawing a very broad nukitsuke, dropping the kissaki way down your back after furikaburi and leaning back doesn't belong in iaijutsu. Same with the large mono-speed circular kiritsuki from way down your back, your kissaki really shouldn't be falling lower than 30 - 45 degrees in waza, any further and you're just wasting time and a real cut is more elliptical than circular.
When you have opportunity watch Peter Ball's kiritsuki, I think his is a particularly good example.
My personal advise would be:
1) Don't get caught up and distracted by trivial technical details.
2) Make your waza meaningful, think to yourself would my technique work against an enemy or am I taking too much time and leaving openings.
3) Practice 'relatively' slowly, but you must retain the definition (fast bits moderate, slow bits slower). It's advantageous to practice slowly because at a slower speed you have more time to think and correct your form while you do it. Beyond a certain speed you're on autopilot and not learning anything from it or improving it.
4) Always practice with an aim to improve something but don't give yourself too many things to work on at once. I would say 2 things maximum and focus on them for a significant time (3 months maybe) so they become part of your form (they become trained in).
It's all here: http://www.youtube.com/watch?v=rdevJzms_Yk
I hope some of the above rambling is of some use to you. I'm struggling with some of the same points so hopefully what I've said will give you some food for thought if nothing else.