it stays relatively fast up through 10th or so level... i think i'm 13th now, and the last three levels have taken about 1/3 of my total play time (about 25 hours).
since there are about 18-20 levels woth of gain in this game, and i'm half way or so, i'm guessing the level up speed has, well, levelled off.
NWN1 was the same way. a 40-50 hour game and most finished around 16-17th level first time through.
they sort of have to do this for several reasons, IMO. one, as you note, to get through the low-level instadeath problem. the second is simply because of the story. you're eventually dealing with powers that are trying to take down a whole city. 5th level parties aren't up to the task of dealing with the higher level NPCs driving the story. this means lots of levels. if the game paced them all a little slower, it would be 100 hours or more of gameplay, which is unreasonable to expect.
really, if you think about it, each fairly major area (e.g. owls well, etc.) would qualify as a separate module in PnP. this whole game would take a year (or more) to get through playing 10 hours a week in PnP.
taks