How to fix the mouse lag on the PC version of Dead Space

It depends on your monitor resolution. Larger monitor resolutions have more pixels (or dots). So, for example, if your screen resolution was 1920x1200 and your mouse was capable of a maximum DPI of 600, you'd have to move your mouse two inches to get to the bottom of the screen to the top of the screen. If your mouse was capable of a DPI of 1200, it would only take one inch to make the same movement on the screen.

In short, as long as screen resolutions keep increasing, mice resolutions will need to continue to increase as well to prevent people from having to move their mice uncomfortable lengths to cross the screen.

Turn V-sync off, usually it's the cause.