So, for someone who has played games all my life, I'm not particularly good. So when I got Ground Zeroes, I spent a lot of time getting very frustrated, but I eventually worked out a method to get an S rank (the fact I didn't discover the run button until after my third time beating it didn't help)
I saw a speed run (where I discovered the existence of the run button) and noticed he killed 2 people. I wanted to do a pacifist S rank (kind of surprised you can S rank with anyone dead... That doesn't seem very Metal Gear) and after about 30 tries over a couple days, I finally pulled it off. Super hyped (/terrified) for (/of) Phantom Pains. Oh, and it's not a SPEED RUN per se (I will never be able to do a speed run) just an S rank, which does mean going faster than normal.
On a side note, I'm REALLY glad I got this game for free. It's not worth 30 bucks.