Best sensitivity settings in Call of Duty

I’ve been playing Call of Duty for a long time and I’ve always been very bad. I don’t understand why. Maybe it’s my sensitivity settings? But how do I set that up correctly? I always leave it as high as possible so I can aim quickly.

Now I started playing only as a sniper, but I’m not fast enough and I miss a lot of shots. I don’t understand why. To me, I’m aiming correctly, but the shot seems to go off?

What would be the best configuration to play as a sniper in Call of Duty online? Or does that depend on who’s playing?