I have tried all these and the mouse is no longer slow, but it still moves more vertically than horizontally. It's a bit bothersome.
I've found a solution !
Step 1: Download Mousex
Step 2: place mousex.exe in your \Constructor directory.
Step 3: Change the sensitivity calue in your dosbox.conf: "sensitivity=200"
Step 4: Change the second-to-last line from "game.exe" to "mousex 100 90 game.exe"