Den. speciosum is actually pretty slow, especially the larger types (like grandiflorum and hillii). But they have to get big before they bloom. So one can wait 10 years or more for a seedling to get big enough to bloom - by which time it is quite large. Not a good species to grow if you have limited space. Lots of hybrids with speciosum that make lots of flowers without taking over the growing area.
I'm reminded of the first time I went to the Santa Barbara show, early in my "orchid career" after attending another show where I had bought a fairly small Den speciosum. The centerpiece of the SB show was a Den. speciosum var. hillii that was about 3 m (9 feet) across, with about 400 spikes. My reaction was "OMG, I just bought a St. Bernard puppy! I live in a condo!!" It turned out not to be a problem, the plant grew slowly and I didn't grow it all that well, by the time that it was big enough to bloom I had sold the condo and bought a house with a yard. But think of the possibilities.