On a show like this, a great deal of the script writing is done by the writing team / show creators and script advisors working together at the planning stage, in which they look at the whole season (easier for them than for other shows because it's based off a book), split the story into episodes, establish the various storylines, spot and solve the adaptation problems and discuss the various options, like what should be included and not, simplified, changed etc. - all with an eye kept on the global season. To give you an idea how long this process is, the show's merely confirmed for a third season and already the creators can discuss publicly where the third season will stop in the story... because they already know the basic shape of season 3. That very long early stage would be the one where most likely Martin himself is most often involved (for this and all his consulting work at many levels, he gets a co-exec producer credit). For the rest, his involvement would be limited to the individual episode he pens, and that's when he would have to work on a deadline, one not very hard to meet as for a writer/scrip writer/story editor of his experience it's a very technical job - he fleshes out a scene-by-scene outline to which he contributed and which is based on his own story. For him, penning this script is similar to a rewrite or editing. That's why he can work fast, not because he works better under the pressure of a deadline.

Alexandre Dumas worked much like modern TV writers do, trying out of a scene-by-scene outline to write novels in record time, rarely going back or making any change to his outline when he encountered problems... with the results you know.

Martin's problem was that there's a whole chunk of his story he decided to tell only in occasional flashbacks, very likely because he judged what happened in those years in the story were neither very important or interesting to tell in full, and they would have been laborious to tell in full. But he realized this didn't work (likely because it cluttered his storytelling and too often he had to stop the story progression for his characters to think back on events from the last five years), and struggled for years to make a proper story out of these events and to adapt his thematic and character developement to include all this stuff, and no doubt he re-arranged the timelines and found ways to rewrite elements of the next novel into an earlier timeline. It's no wonder it took him so long, and that the books are slow and don't bring major developments - most of this stuff Martin initially planned to refer to only. So yeah, the last books were not of the quality of the previous three, and it's rather obvious why Martin wanted to skip ahead to where the storylines end in the last two books. That's fairly bad planning, and probably he should have seen the gap was too long and too much back story would have to be explained later for that gap to work, but the writing itself wasn't much affected, it's the storytelling, the story matters, that were not up to his usal level. Martin didn't quite met well the challenge of turning stuff he wanted to skip into something as interesting as the events he has planned to tell. It's not surprising this happened, with novelists insisting to work on this overambitious, bloated scale, juggling as if it was all a single giant novel with story material worth 10 or more regular novels - all "in development". Most of them (Jordan, Erikson etc.) ran into similar structure and pacing problems down the line. They would be reasonable to deal with or correct at novel length, not so easy when those problems are caused by what you've published years ago and you have all your planning ahead to put in the balance in the way you solve them. Martin struggled madly and caused big publication delays, with mitigated results. Faced with a similar (and later admitted) problem, Jordan decided not to rewrite and that gave us Crossroads of Twilight.

It's with the next ASOIAF novel we'll really know if Martin's lost his touch or not - and if it was worth the last two novels and the all those years he took writing them (though admitedly if not for my desire to watch the series leading to a first reread of books 1,2,3 after which I finally read books 4-5, I too had the ASOIAF books in storage to return to only when and if Martin finished the whole thing. My gripe with writers like Martin and their overambitious projects is that with the publication gaps - in Martin's specific case they're indecent - they pretty much force you to do multiple rereads of very long novels that aren't really worth devoting so much reading time to. Stuff like ASOIAF, WOT etc. become a very time-consumming form of entertainment - which is why aside from these two I'm stuck with I've virtually stopped reading ongoing series)