Definitely this. I've been riding a lot of hills recently on my fixed gear bike, including many that I thought were way out of my league running single speed. I've found the best thing to do is focus on steady breathing and not elevating my heart rate. If I feel my heart start working harder, I slow down. I actually tend to climb faster when maintaining an "all-day" pace like this than I do when I try go aggro and just muscle up a hill.
I'm confused. If you slow down too much you'll eventually slow down and then fall down... at least that was my impression of fixed fear riding.