Slow Writing

I have a persistent problem with misconception. I can't possibly be the slowest writer in the world, but just when I think I've poured streams of words onto the screen, I go back the next day and re-read it and it amounts to about half a page -- a whole page at most. How is this even possible? My writing sessions generally last three to four hours. I allow no distractions. What is happening? 

Yesterday not being a writing day, I pulled up my latest work and figured, well, this'll be a nice, leisurely stroll through the narrative. I was done reading it in about five minutes!

Maybe I unconsciously spend much of my writing time just thinking. I don't have an outline to follow, so I am constantly plotting, deciding what comes next. But I must be a slow thinker, in addition to being a slow writer.

Part of my issue, too, is editing while writing. A bunch of people advise just getting the first draft done, regardless of obvious mistakes, poor word choice, time jumps. I can't. And if I use the same word to describe something somewhere else in the paragraph, I have to find a different word to replace it. I can't abide, "She moved the thing and then found that the thing didn't belong where she put the thing." WTF? I spent way too much time trying to find a synonym for mattress (there isn't one that's even close) because the first sentence in the paragraph mentioned that the MC picked the wrong day to move the mattress, but she went ahead and moved it anyway. Moved what? A futon? (no.) A pallet? (obviously not.) A "bedstead"? Who in the twenty-first century even uses that word?

I suppose it would be better to just use "mattress" a hundred times and fix the manuscript on the back end, but while I'm not an OCD person, my mind won't let things like that slide. I need to be satisfied, at least mostly satisfied, with the last paragraph before I can move on to the next. But at this rate, the novel won't be completed for another year.

My slow pace is particularly frustrating this time around because I just want to be done.