Deleted over 1,000 fonts and it fixed it! Before, in ArcMap 10.5.1 64bit, it would take exactly 1 minute and 50 seconds for the layer properties box to appear after double-clicking on the layer in the Table of Contents. Now it takes 5.3 seconds. The grid properties and text properties open very fast now too! I am in heaven! I've been putting up with this problem for months, even in the older versions because I use CorelDraw and had WAY TOO MANY FONTS in the C:\Windows\Fonts folder. First I backed up all the fonts to a new folder, then in the C:\Windows\Fonts folder, I selected all the fonts that started with "ESRI ..." and then inverted the selection, and hit delete. Windows will not let you delete the system fonts, so just hit okay when the warning box comes up and the box that says to quit telling you. I went from over 2,000 fonts in there to 365!
IT WORKED.
I think ESRI should have it's own font management application to go along with ArcGIS, and a font folder just for ArcGIS to solve this problem.