Why Am I So Slow?

I feel like I am writing at a snail's pace!  In actuality, I am not doing terrible; it's all about perception.  When I was in high school, we had to write drafts for our English papers.  I was the annoying student who never wanted to turn in a paper with errors.  Nine out of ten times, my first draft was perfect, and I didn't have to do another.  I think I am having that problem with my novel writing.  I want everything to be perfect on the first draft...impossible!  When I think of new ideas or plot twists, I keep going back and adding them.  I think what I should be doing is writing without stopping, and making notes about the new ideas so I can add them later.  I'm pretty sure that's how Stephen King writes, but don't take my word for it.  I guess if I'm ever going to get through the entire novel, I need to stop trying to be perfect and just let it flow.