After moving several client machines to Win10 we see slow message boxes on all these clients. It turns out that all types of msgbox are drawn instantly as long as no title text is used. The delay is round about 1 second.
instant draw
msgbox "A"
msgbox "A","","?,YESNO",a$
slow draw
msgbox "A","A"
msgbox "A","A","?,YESNO",a$