One of my students is practicing Le Banquet celeste by Messiaen at the moment. The other day I was listening to him play at the church.
Everything was going smoothly for him - the legato was perfect, the drops of rain in the pedals were quite convincing in this acoustics, breathing at the end of legato signs let me hear where the next phrase began. I think he even started to appreciate the modes Messiaen was using in this piece.
But one thing was missing - the tempo was too fast.
Yes, he played very slowly but not slowly enough.
You see, the way to determine the speed in this piece is this:
The rain drops should NOT connect into a melody. If they form a melody in your head, it's too fast.
Also try to focus your gaze on each note separately.
Treat it like meditation. That's what it is.
Organ takes you beyond reality.