.. your comparison is quite off and unfair.

Martin hardly writes 500 pages/year. He gets 500 finalized pages of ASOIAF a year on average in his better years, which isn't the same thing at all. He writes tons more than this to end up with those 500 finalized pages.

Sanderson won't do much differently, in the end.

Sanderson's 10K night is merely a first draft. Those 10K are not in any publishable state, full of typos, missing elements, errors of details and ackward writing (that's how he described his first drafts, anyway).

It's just his process, and it's hardly unique among writers, great and bad. He likes to throw the whole story on paper in rough draft without questioning the writing itself much before he steps back and begins to rewrite, polishing his stuff in further and further drafts, and usually he also cuts a lot of stuff out in rewrites - then he works with his editor.

That's how he works, that's how a lot of writers work. Others plan and outlines massively before they actually get to write (which I understand is how Martin prefers to work, and how he ended up having to throw away a great deal of very advanced material when he realized he had to change his plans as they didn't turned out as he expected), Sanderson prefers to work a lot on the world building and characters and on the shape of the story, then rather than outline the story in details before he starts writing, he prefers to rough draft it completely, often leaving a novel in this state for years, before rewriting it (as he did with Way of Kings). Jordan had yet another different process. He planned and outlined a lot and mostly in his head, then he wrote the scenes out of order, starting with the most important of a story arc (though not necessarily the big events, it was often important scenes of character development and similar turning points), or those that were the clearest in his mind at the time. Some of them he worked to near final state right after his rough draft - RJ did usually do 7-10 drafts (for AMOL some of those happen to be the scenes BS almost used as is, for instance in the prologue, Rand's epiphany, Moiraine's rescue, the epilogue and so on) , for others he merely wrote important dialogue, returning to write the full scene around the dialogue much later (likely because what's being said was the most important, while the setting itself might still change). Gradually all the pieces fell in to place and the secondary scenes got written.

Sanderson prefers to get the story out of his system and then fix the storytelling problems that arise from this rough draft, and when the book is more or less structured, he works on his prose, on rewriting scenes, and globally on refining the book.

There are several months of rewrites, polishing and rewriting ahead before you get to see any of this stuff.

The pace Martin keeps isn't a warranty of literary quality. It's merely a sign that his writing requires a lot of time and effort on his part. There are tons of writers who write much better prose than Martin who write much, much faster than he does.

"Agonizing" over a sentence for days would probably not change much of anything for Sanderson. He's just not a very literary writer. There are limits beyond which it's a matter of talent and more time won't change much (though if you judge Sanderson on his WOT books alone, yeah time was a major factor). Martin's prose is better for sure, though I would hardly say he comes close to the great novelists'.