Time moves differently when you're writing. You enter a portal, a wormhole. You travel backwards and forwards and then stand still.
Hours may have passed; you either feel every second, or none of them at all. You've experienced entire lifetimes, seen your worlds, your characters born, live, and die.
It's different from reading. This isn't immersion into a world already created--you're the one doing the building.
Mountains rise in one scene and fall in the next. She has blue eyes, then brown, then she's a he, then he's gone. It's happening one at a time, and simultaneously. Your fingers can't seem to type fast enough, your mind is whirling, it's all coming together, and then!
It's done. You've lost it. Time drags. How could minutes move so slow? You have to put in the time, sit in the chair. It just isn't working. You don't dare read what is already done. You have to look ahead before you go back--you need the word count, you need the sense of progression. Editing is your enemy now. How could you think this would be easy?
But it's worth it. When the worry falls away, when the mistakes and typos are minuscule blemishes on the face of something truly great--that's what you're writing for. That's your portal, your wormhole to another world. The rest just falls away.