The "10 minute" description actually originated with one of the very earliest versions of DD (back when it was still really only known on its local forum). The game was much simpler back then, and it took me about ten minutes per test run (eased along by the fact that I knew all the math and whatnot). It kinda stuck from there.
Nowadays, I find individual runs do take longer, but I usually consider myself a slower gamer than most -- I also suffer from alt-tab-itis a lot.
What we're really hoping, though, is that the interface changes + the level of feedback the full game gives now will make some of the tedious tasks more streamlined, and some of the dungeons have actually been made smaller to provide people with shorter game options.