Have you tried not updating it every frame? I seriously doubt your score will change that fast (or that the player will even notice).
I usually go with something like this:
//somewhere in your gamelayer's init:
And that’s only because the score is tied to the elapsed time (in milliseconds) in my game, if that wasn’t the case it would work just fine with an even lower refresh rate.
PS: also, I might be wrong, but I think they improved LabelTTF’s performance in v2.2.2, so maybe you could try upgrading?