LrC 12.3 High CPU Usage when merely displaying a panorama; also Expand Stack doesn't work

This just started happening since the upgrade to 12.3.

Windows 10 22H2 with all latest updates

CPU: i7-4790K @ 4.00 GHz (not OC), 32GB RAM, 6TB Disk

Graphics: NVIDIA GeForce GTX750 Ti with driver version 527.37 (latest available)

I merged 4 images into a panorama (image 46 below):

If I am in Develop mode and have a non-panorama image selected, LrC uses almost no CPU in the background.  On changing the selection to a different (non-pano) image, there is a CPU spike for a few seconds.  However, if I select a panorama, CPU usage in LrC jumps to 65-70% for 10-15 seconds then drops to 20% and stays there for 60-90 seconds, causing my entire system to become sluggish and jerky with significant keyboard and mouse lag.  CPU consumption remains high even with LrC minimized.

Here's a screenshot of CPU usage from Process Explorer for the LrC process:

I checked driver versions and saw I was a bit out of date, so I upgraded to the latest available version.  The problem persists and may actually be a bit worse.

Note: This does NOT happen in Libray mode.

Additional note: I had the flashing white screen issue after the update to 12.3, and had to disable the technology preview.  However, I no longer see the Preferences/Technology Preview tab.