Quake2World picks up input settings from your environment via SDL. If you have mouse acceleration enabled in your desktop environment, it will also be enabled to that same extent in Quake2World. Mouse acceleration is a pesky little setting that often makes your mouse-input feel too fast and not very responsive while using slow mouse movements.
Here are is how to disable it in the usual major platforms. Be cautioned, your mouse will most likely feel slower in both your desktop and in your games.
Windows:
Create a new .reg file, and put this in it:
[HKEY_CURRENT_USER\Control Panel\Mouse]
"ActiveWindowTracking"=dword:00000000
"DoubleClickHeight"="4"
"DoubleClickSpeed"="500"
"DoubleClickWidth"="4"
"MouseSensitivity"="12"
"MouseSpeed"="1"
"MouseThreshold1"="6"
"SmoothMouseXCurve"=hex:00,00,00,00,00,00,00,00,15,6e,00,00,00,00,00,00,00,40,\
01,00,00,00,00,00,29,dc,03,00,00,00,00,00,00,00,28,00,00,00,00,00
"SmoothMouseYCurve"=hex:00,00,00,00,00,00,00,00,b8,5e,01,00,00,00,00,00,cd,4c,\
05,00,00,00,00,00,cd,4c,18,00,00,00,00,00,00,00,38,02,00,00,00,00
"MouseThreshold2"="10"
"SnapToDefaultButton"="0"
"SwapMouseButtons"="0"
"MouseTrails"="0"
Then, of course, execute the .reg file.
X11-based (Linux/*BSD):
Download the xset utility if your distribution doesn't already have it (most do) and add xset m 0 0
to your X11 start-up script or equivalent.
Mac:
http://mac.softpedia.com/get/System-Utilities/iMouseFix.shtml
Download, install, and run – self explanatory.