I am currently working on illustrations based on Turangalila.
I did not know when starting this that Messiaen had increased what was normally in four parts, to a symphony of 10 parts. That; its length and the number of instruments, 122, make it formidable. Its concern with birdsong, Hindu thought and other religions add to its complexity.
It took Messiaen three years to complete. That seems to me to be speedy for what it is.