This bug showed up in 10.5. It is fixed in 10.7.
ArcMap 10.5.1 Properties slow
The patch was posted recently. I HIGHLY recommend trying this patch!
There are patch versions posted for both 10.5 and 10.6.
ArcGIS Desktop Dialog Initialization Performance Patch
This is a long-needed usability fix! Hurray!
THANK YOU VERY MUCH.
IT HAS FIXED MY PROBLEM.
