Okay, so I just found out that I have been playing Diablo 3 until Inferno with a handicap.

I just tried lowering the resolution, and the performance became super smooth. My DPS also increased in reality. Apparently, when the lag slows down your movement and attacks, it is reflected in the damage. But the best thing about it is that vault actually works. Apparently, I was seeing the attack animation after it has happened before this change, so I had to predict when attacks were landing instead of reacting to them, thinking the whole time that it was the same for all players.

I just finished the Spider Queen in 1 try after lowering res. It was so easy it was funny.

Thinking back, I could have saved dozens of hours had I done this from the beginning. The game looks much smoother and nicer as well.