What about calling it "learning curve", "attention span" or maybe "attentiveness". It's still just a way to measure the exp cost of each new class, but (to me) it implies that someone who pays more attention can pick up things with relative ease and thus "learn" more quickly than someone who gets distracted easily or that has a naturally slower learning process. Some people just need to look at a book and get what it means, while others need to read it a couple of times until the implicit message drops on them.