Don't turn mouse dpi down - that is silly. Turn mouse dpi UP and turn ingame sensitivity down - that way you get all the benefits of the higher resolution and tracking from the higher dpi mouse setting, but stil have the ability to control speed with the in game sensitivity setting.
As for what helps with online fps, there are a few things:
- practice practice practice - the more practice you get with a consistent setup (ie. don't keep changing game settings, controller configs etc) the better you'll get
- low latency internet - in game latency effects time taken to see other player. The player with the lower latency connection has the ability to see other player first, and so has the abuility to kill first
- reaction time - the quicker your reaction the better chance of shooing the other player before they shoot you.
- suitable hardware - your hardware needs to be able to run the game you play. It needs to keep the lowest FPS above the refresh rate of your monitor.
- quality monitor - buy the best monitor you can afford. The higher the refresh rate and the lower the input lag the better. Those with higher refresh rate and lower input lag have the ability for higher accuracy, and higher accuracy = win
- quality mouse - similar to a quality monitor, you want a mouse that can provide you with the highest accuracy as possible. So it needs high dpi and high tracking capability. The higher the better and the more accurate it can be
That's pretty much it. The more of the points above you have, the better you can be.
But of all the points above, #1 is the most important - without practice, you will never improve.
You need to learn every map and every nuiance, bug and glitch of the game to give you the best chance of staying alive long enough to shoot the other players. And you need to learn how to fire the weapons for the most accuracy. It needs to become second nature, and the more natural it becomes for you, the better you will be.