There was some slow-ish play during a few of the planning phases, but overall the reason the game too so long was that it went 30+ rounds. Hanma went through all 3 of his Wizard's Towers, all of his Mana Crystals, Nearly all his attack spells, etc. I think Drefan cast all his plants, lost his tree, lost most of his plants, all his walls, and his flowers, too.
It was only conceded when they determined that Nick still had a Dissolve to put on his Mage Wand, while Drefan did not, and nick could therefore, eventually, remove all Drefan's armor and win the inevitable punching match that was going to come when they both ran completely out of cards. Out of their 40+ books, they'd both played at least 30 spells.
Also, exhaustion. You wouldn't think a boardgame like this could be that taxing, but there's a reason Magnus Carlson is in such great shape.