I have been ridiculously spoiled by Brandon Sanderson's seemingly inhuman ability to write.
Even given that though, Martin is pretty ridiculous in his pace. It doesn't help that he's completely incapable (and he's admitted this) of writing anywhere but on an ancient DOS machine at his house. No laptop on the road. Given the sheer amount he travels it means there are these long stretches where even if he had significant downtime (on planes etc) he just doesn't write.
And frankly, it's inexcusable. He HAS lost entire manuscripts or large sections of them before because of his reliance on that single, ancient, machine.