You need to do this:

1. Check your current mouse speed:

   xset q | grep -A 1 Pointer

   You can see string like this:

   Pointer Control:
   acceleration:  2/1    threshold:  10
   

   2/1 - how many times faster the cursor will move than the default speed.

2. Set new value:

   xset m 1/2

   1/2 - It's my settings. You can try other values: 1/3, 1/4, etc.