I also have the same problem since ( a month ago) I installed win10-64 on my home PC…
anytime RT tries to build a different GUI environment (first opening, opening the edit panel after double clicking a thumbnail, opening the preferences panel, etc) it takes minutes to open.
I only tried for a while (checked the situation with big folders etc mentioned before) then left home for a month so I cannot give more details …
The RT version was the (then) latest 4.2.8xx (compiled by sguyader) and I used the GTK2 variant …