Jo-ha-kyu in suburi; or, 'how fast should I swing the shinai?' (answer - every speed!)
Watch this Noh actor for the johakyu in his retreating steps.
Suburi, as we know, is a very important form of solo training for the development of efficient movement in Kendo.
When you are practicing, it might become confusing as to how fast you should swing. When you do suburi or kihon waza at training, while you are warming up it might be difficult to find your tempo. There is a very common tendency to rush, born of two things 1) the influence of watching people around you move quickly, and 2) trying to please sensei and her/his admonitions to do your best!focus!move faster!etc.
In these situations, whether by yourself or training with others, using the principle of "jo-ha-kyu" can help inform correct and powerful technique without excessive force or muscle power.
Jo-ha-kyu has been well-described in English on the internet. This article is especially good (as is the blog it is taken from). Johakyu appears all over Japanese culture, from the movements of the Noh, to the way the we beat a drum in the dojo (don.............don........don......don....don...don..don-don-don-dondondondon..... don!). We even use johakyu in the three steps between bowing and sonkyo.
With suburi, you should use johakyu to begin slowly and then accelerate through the back-swing to the cut, and finally tenouchi. It is absolutely essential that there is no stopping in your movement. This is one of the bad habits that johakyu can help eliminate: the tendency to pause slightly at the top of the swing.
Johakyu means basically that you don't swing fast or slow, but fast and slow and every velocity in between. Explore this movement and timing deeply. This is why we do suburi. It is our personal research time.
What I hope you will find is that your swing becomes easier, more accurate and more powerful and that you use less effort. The speed of your cut when you use johakyu timing is constantly increasing. This makes it unpredictable for an opponent. Hence it is more than just a training exercise, it has martial relevance as well.
When you return to the dojo, use this timing to warm up your body and ground your technique during the first few rotations of kirikaeshi and kihon waza.