ok... I got some steps to reproduce the issue to get into this "high CPU usage" state.
1. Launch Kyma
2. Open any sound file (double click from the browser), I'm going to use "An Overview.kym"
3. Play (spacebar) any sound that has a VCS, I'm going to use "Cross Vocoder noise+oscil->VOICE"
4. Stop the currently playing sound (spacebar)
5. Close the VCS (native window 'x' button)
6. Now click anywhere on the sound file's window native title bar (clicking on prototypes title bar will do the same thing).
7. Notice CPU usage for Kyma at 100%
8. Click anywhere inside the sound file's window.
9. Notice CPU go back down to 10-20%
In any case, I discovered this because I noticed my macbook pro's fans were going crazy, and I must've just moved/dragged the sound file window.
When the application loses focus the CPU goes back down as well, so it's an edge case for sure.