I think its a lot more complex than just one thing, I was on my 50 Con at one of the strongholds gaining XP like no tomorrow (750+ a kill) and the spritbond wasn't going up all that fast.

I ended up going back to town to craft on my 38 Wea, decided to craft a bunch of cloth for a meager 150XP a synth and the spiritbond went up much faster.

I had the spiritbond choker on in both situations.