Ajira* went through the Circle day before yesterday and I noticed that the time stamp of the departure save was exactly double the time of arrival**. I found it surprising that the seemingly short slog through tower and Fade should have taken the same time as everything before taken together: mage origin, Wilds, Ishal, Lothering, plus a quick trip to Denerim for visiting 'Weylon' and the Wonders of Thedas.
Slothy took almost half an hour because I didn't have the energy reserves to try fancy new moves; for the most part I relied on Fireball kiting. Uldred also took a while, more than five minutes. But even that can account only for part of the long time spent in the tower.
Part of the reason is probably enemy/event density, or lack thereof. It is already lowish in the Tower but in the Fade it's extremely low, coupled with the fact that all enemies there are nerfed to make them easily soloable for an average player. Apart from Slothy there are only two fights that pose enough challenge to get my full attention: the twin golems after a mousehole, and - after another mousehole - the duel with Vereveel, if she gets the drop on me instead of the other way around. That makes for a lot of running around with comparatively little 'eventfulness'. By comparison, Ishal is all meat and no fat.
And the Circle is one of the short quest lines. There is a bit of backtracking (in the Fade) but apart from that you can start at the beginning and just hoover XP and loot until you get to the end. The same for the Ashes and Redcliffe which are even shorter, about the same as Keep, Honnleath and RtO taken together.
Denerim, Orzammar and the trip to the Dalish take forever because of all the running around, booking quests, investigating, errand running, politicking, shopping, playing Agony Aunt, retracing one's steps, etc. pp. Even the Deep Roads are fairly benign in comparison, there's only a small bit of looping and backtracking.
*) nightmare solo fun run mage with Vestments, Sash and High Regard (plus Croesus cudgel)
**) arrival 2:54, departure 5:48