clarry: I think it's only natural that the first time will take much longer. You don't know the place, you don't know the codes, you don't know where all the enemies are and what they are capable of, you don't know where all your goodies are, you aren't familiar with the AI and therefore don't know that you can just run up to them and smash them in the face with the crowbar...
Definitely there will be plenty of sneaking and exploring around in that case.
Indeed. Although the 2nd time I played Deus Ex, I actually spent nearly 1.5 hours on that 1st level because this time I go everywhere. You know, exploring and sightseeing :)