Yeah, there are plenty of authors who output a high volume of work, but few of them have created a world as complex as Martin's. That's not excusing his slowness, of course. I think he's just gotten lazy as he's gotten older, and his story has gotten too convoluted even for himself to handle. That said, when you read other fantasy series, few of them even come close to having the depth that Martin's world seems to have where so much of the history feels vivid and mapped out.
As for Rowling, the Harry Potter books were much, much more simplistic than anything Martin has written. I'd use someone like Joe Abercrombie, who writes epic adult fantasy but does so at a much swifter pace. Abercrombie's work isn't quite as good as Martin's, but his best stuff is at least close. Then there's a guy like Brandon Sanderson who pumps out an absurd volume of work at a somewhat lower quality. I actually like Sanderson's work a lot (Mistborn and Stormlight Archives are both great and unique fantasy works), but there's no denying that it's nowhere near as well-written, clever, or deep as A Song of Ice and Fire.
I tend to agree with Rich's observation that Martin has kind of written himself into a corner. He stacked up so many subplots and interesting characters and scenarios that it's very difficult to work them all out in a timely and satisfying manner.