Thanks for your response. Although, disappointingly after following your suggestions there is no change at all.
However, you say that you have seen no problem, i.e. it is not a bug. Therefore, I want to talk through step by step exactly what I did this time, in the hope that you can see an error I'm making.
1) You sent a link ("using this VP") to the latest update, I downloaded and used (although same version as one previously used).
2) I added '1hr 30min' mp4 file to bottom screen panel.
3) Clicked on 'effects', 'Speed change'.
4) Changed to 90%, and ticked both boxes as you suggested.
5) Then clicked on 'full menu' at top left of screen.
6) Scrolled down to 'Export', then across to 'video file'.
7) I then saw the screen 'export file settings'. In this screen I left everything as it was (preset is 'default').
8) Then I clicked 'create', and it went into 'export queue'. The time it took to export was the same time as the video itself.
After exporting I played to see it is the same as before.
To repeat myself, I really need this function to work. Therefore, I would really appreciate you finding where the fault is.
I await your reply.