I can speak for myself that my dive times to 45-50 meters are about 1:35: to 1:50. I did a lot of "slow" dives to 45 meters for about 1:50-2:00, but I hope that is the past.
I am aiming to shorten my dive times to 1:30 1:35 for 50 m dive, since I beleive the good "speed" of the dive is very important for performance.
But, anyone should have his own tempo and deal with his own numbres.
For instance, in Ibiza Herbert Nitsch did a -86m dive for about 3:10 or something. Greek guy, Manolis did-81m dive for about 2:20 something (fast, ha?). I was watching it live and later on video, and I must say that Herbert Nitsch is one slow diver. But, I guess that this is the tempo that he choose because of saving oxygen... don't know
For the shutting down engines and sinking, I do that at over 30 meters, sometimes over 35m. I am negative bellow 15 meters but I would loose a lot of time if I start to sink earlier.
My descends last a bit longer then ascends somewhere about 55-45%.