Hi Alan,
An upgrade can be delayed or avoided by not clicking on “Update All” when it rolls out but at the moment there is no publicly available roll back procedure or previous version available to download. I can recall, I think, one occasion when an update had sufficiently serious widespread consequences that an immediate further update effectively rolling back for particular hardware was released.
That hasn’t happened in relation to either Builds 209 or 216 (Release notes here) however and your report is the first (at least that I am aware of) where this problem has occurred.
Is it possible that this CPU activity is (re)analysing of your library ? If that is occurring you would see a circular processing indicator to the top right and clicking on it would give a progress indicator of analysis.
Otherwise it is likely that @Eric can determine what the issue is by reference to your logs and he will be along shortly to investigate further. It will assist him if you could set out details of your system and network as per this post.