Okay I have more to add. In one instance I was able to get it to toggle from fast to slow by selecting Document->TOC. It didn't speed up again with the TOC closed.
I checked the output of "top" and found my system to be 100.0% idle. (!) And then I got it to go from slow to fast by resizing the LyX window. Which was meant to see the output of top while scrolling :) I couldn't reproduce it the same way again!!! Next time, I'll make "top" always on top :) Have fun, Darren
