Thank you for the test model. I tested with the following combination.
EnableGL = True
EnablePreview = True
EnablePreviewCache = False
And now I can also see a lag when opening the dropdown. When the Cache is set to false, then we do not cache the previews and they get always regenerated.
Can you make one more test for me. How does the dropdown work when both of these are set to false and then restart Rhino?
EnableGL = False
EnablePreview = False.