| Here are some reasons I think may be the reason why I can play through solo games really quickly:
If you've played through the game many times, whether or not you are playing solo, you will play the game faster - since you are more familiar with the ins and outs of the game. You will instinctively develop routes throughout the game that maximize time. I rarely rest during solo campaigns and I stretch my characters out as far as they can go before seeking healing.
Also, when playing solo sometimes you can skip enemies - because your level gets inflated early, a lot of the lesser opponents won't give experience when you kill them (most notably the Aurilites (but not the golems) and the inhabitants of the Underdark caves).
From experience, you will generally know which items you want and where to find them so you don't have to go around opening everything and exploring needlessly.
I don't read the text anymore (except the replies of course).
Finally, playing solo decreases the pre-battle prep time signifcantly - this is self-explanatory.
In the end, I can't really say for sure if soloing plays faster than the regular multi-character campaign since the last time I played with a big party was when I first got the game (that would be one run through normal and two HoF runs), and it was slow mostly since I was still getting used to stuff. I think that if I were to play a large party again it would still be as fast, but I won't really know til I try it again. Another note, playing HoF will significantly increase the amount of time it to complete the game.  | 
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