I removed the now unused page styles, and that helped with responsiveness. But still there was much accidental formatting, like words with errant font sizes, etc. When I removed all formatting and made everything one paragraph style the problem was immediately solved. Updating the table of contents is now almost instantaneous, saving takes a few seconds and only takes the cpu to 2% for a couple of seconds. I am certain that the formatting anomalies was the cause of the problem. Removing all formatting does create a job reintalicizing titles, recreating lists and indents, etc, but it is worth it.
Thanks.