If I have the time and can get comfy and focus, I can probably write at least a couple of thousands words a day. That hasn't happened in a while, though.
I am very familiar with the "binge writing" LC describes. About 20 years ago, in college, in our noisy students club, someone had set up a computer asking us to write a story. So I sit down, and two or three hours of full concentration later I had a complete story, from scratch. An idea that just came there and then. No idea what happened around in that period.
To come back to OP's question: creative writing and technical writing (dissertation) is something very different. Language use is different, thought processes are different. The first you make up, the second you use and process facts and research results. I'm not surprised you get so different results, and as another poster indicated, this may go both ways.
For me, creative writing generally goes much faster as well, mostly because I can just make it all up as I go, all I need is a good character and a good setting, and let the character's character do the work. Role-playing with yourself.
Huh, I'm different - I'm much faster when it comes to non-fiction because facts are facts. I don't have to decide how the stuff I'm writing about works - I just have to report on the way things are.
Sit me down to write fiction, and I'll write a whole bunch of alternate versions of how things could happen before I settle on one that I can still respect in the morning.