I've tried Orbis for the first time and noticed that the speed at which difficulty increases is much lower than in ffh2 (50 turns). I'm near the end at the tech tree and I'm still at monarch, while in ffh2 I'd have been at deity a long time ago. Considering that I've started at chieftain and by the turn 860 (marathon) I'm at monarch I'd say it increases every 200 turns or so, is that correct?

Perhaps increasing difficulty scales with the game speed?

Could be, but I'd say it's a bit too slow. It goes through the lesser amount of difficulties but maybe that was the point.