Basically here's things you need to know and get used to.

Bullet delay: The bullet isn't hit scan, it takes time to get to the target so you have to lead.

Awareness of guards: Guards seem to have an awareness circle around them, less behind than ahead. IE you can shoot someone per say 20 feet behind and 20 feet in front, and the guard will see the one in front, but if it were 10 feet behind and 10 feet in front, he would see both.

Civilians: As far as i can tell you can kill as many as you want. Almost all civs wont be a hassle, except for one woman, and one old man. The woman is okay and is pretty avoidable, but the man will mess up many of your speed runs. For stealth speed runs, when your not doing anything and waiting, shoot bodies of the ledge to prevent the old guy or woman from seeing them.

Leveling: The more levels you earn, the more fast your runs will be, due to faster reloads and re-chambering rounds. To level up fast get all the achievements and grind, pretty soon I will make a guide on how to do it very fast.

Breath: Never ever use breath. All it does is slow down time in game, delaying events, but not the actual game timer. Trust me, it's a cancer to speedruns.