There is a difference between demonstrating a kata / form and studying a kata / form.
If I am testing, or demonstrating a particular arts version of a kata / form, then I would use the timing as taught by that art... the fast parts fast and the slow parts slow.
If I am studying a kata / form, that is different. And changing things up is very good.... take your toys out of the box and play with them. I like to start by doing everything at a very constant, very slow speed. This helps to study balance, structure, form, placement issues. Then you can work on timing variations.... do the slow parts fast and the fast parts slow. Do the first half fast the second half slow.... however you want to break it down.
Typically a block/counter subsequence should be faster than the rest of the rythm. So we try to envision that actual fight while doing the kata, and then it becomes more natural to group movements and change rythm.
Sure... and I agree.... but.... you can also do the opposite. Typically, you have a block counter sequence that is grouped together, then some transition, followed by another block counter sequence that is grouped together..... Take the last few moves of the first sequence, group them with the transition and the first moves of the next sequence. Practice and study what you need to do in order for that to become a natural sequence. Many times, it leads you to find things in your kata / form that you never saw. If for some reason, you find nothing here (you got very lucky in this case.... there is always something to find), the time you spent working on those movements and transitions will only help you do your kata / form better in the original timing.
Of course I also like to change other things as well. Instead of imagining the opponent in front of me punching, I sometimes imagine him much closer, grabbing a wrist, or trying to initial a grapple. Sometimes he is to the side or behind. Sometimes he already has a hold on me, other times he is trying to establish his hold. Sometimes he is far away, sometimes he is close in.... sometimes he is retreating, other times he is charging in.
All these different changes, allow you to really look at your kata / form. You will see new things to work on and find new applications of the movements. By making these changes in your study, many times you realize that that "extra" part, actually has a use and a reason.... you just could not see it before.
