How odd. I think I've heard similar things from other magic-using rifters.

I wonder if it's just that Thedas naturally doesn't have magic flow as fast as other worlds? If that's the case, I would guess the Fade as the cause of it. Another possibility is that something occurs when passing into our world as a rifter that limits your spellcasting ability...but I've never noticed rifter mages falling behind Thedas mages in ability?

Solas would know more about all of this, he's the expert. Unfortunately, he's presently absent.