When recordmydesktop runs out of CPU (esp. with on-the-fly) it begins calculating the delay between frames incorrectly. That is, it drops frames appropriately since it cannot acquire them at the rate it wants, but then it records those frames as if nothing in between them had been dropped. The effect of this is when the CPU maxes out, the video that results goes into a sort of 'fast-forward' mode in places, like time-lapse photography. It's impossible to slow down those places as well, since recordmydesktop isn't recording just how long it got delayed between frames. Ideally the video would be jerky but not sped up, whereas currently it smoothly jumps between frames at an uncalculatably accelerated rate.