And if that slowness presents no real world, in use problem? Then what?
I have used Olympus om-1 and om-2
Nikon F80 and F100 both underwater where I didn't find the viewfinders to be a limitation to well composed images.
And a couple of rangefinders above and below.
Sorry I did not get the first question.
Then you must be doing a much better job if you can see very well from your F100. I tried my Nikon 8008s (HP viewfinder like the D2X) in a Stomm housing, but gave it up because the Nikon F3 with the action finder was just a much better setup for U/W shooting.