If this had been hitting lots of users I would expect you would have heard the screams here. So you are likely going to need to troubleshoot this, and you should make lots of progress following standard troubleshooting steps. (Start at “Help us help you” at the top of each DUC web page.
Have you tried trashing prefs? (The first thing for any problem). Use Peter Gates’ utility
http://petegates.com/pg-pt-prefs.html
What do you mean exactly by “with no Plugins running?”. How have you disabled/defeated plugins? You may not actually have stopped the CPU load.
This is likely very plugin related. Are all your plugins up to date? Next troubleshooting step is likely to try removing all .aaxplugin plugin files, trash prefs again, (Pro Tools will magically restore the standard plugins) and seeing if things speed up again.
Looking at Pro Tools CPU meters is often a waste of time, but in this case may be interesting to see what is going on, now and after you remove all the non-standard plugins.
Still having problems test starting with a new empty session from a new macOS admin account. (That excludes some extreme account preferences and other corruption).
Did you jump off the cliff and do an insitu macOS upgrade? (A bad idea) Or was this a new clean macOS Mojave install... and can you boot back to the previous install to A/B test this? If you did an insitu macOS upgrade and you cannot make other progress then do it all properly, grab a spare disk partition or drive and do a full clean install of Mojave onto that with only the minimal drivers, ILM and Pro Tools and test that, add third party plugins in batches while testing more.