Also, part of the reason the Tyrion chapters became harder to read in ADWD was because it was becoming more and more clear that Tyrion wasn't that likeable anymore. And in fact, he may have never been that likable on his own to begin with.
A large part of Tyrion's appeal (aside from the hilarious lines he got) was that all the characters around him were such 1-D asshats (Cersei, Joffrey, Pycelle, etc) that we enjoyed him being the one character who regularly humiliated and opposed them and could get away with it. His nasty behavior to them was welcomed and appreciated.
In ADWD he's no longer around them, and the folks he's nastier to are more likable and/or have done nothing to gain the audiences' contempt. So you begin to realize that this is probably how he'd come off in real life and it's not so likable anymore since he see him being equally a jerk to normal folks.