Thanks Szalam, unfortunately the link I need on the page is broken...
This necessarily changes the behaviors of starting and stopping previews. Learn more about the interactive performance improvements that made this possible in this article.
I found this:
unable to stop a preview before rendering finishes, or long delay before you can stop a preview
We have received many reports of a problem where After Effects CC 2015 (13.5) doesn’t respond to mouse clicks or keyboard input while a preview is rendering and caching frames. Once the preview is fully rendered, After Effect will respond to input again. In some cases, After Effects will respond during the preview, but only after a long delay.
This problem doesn’t occur in all compositions. We are investigating this problem.