This may seem very similar to the discussion from 2310 x64 The game is slow. But it is not, I faced slow mouse issues on the choosing a starting location for the past 4 releases and in Alpha 8.
Steps to reproduce.
Start new map.
Move mouse to a location you want.
Then move to another location.
The box choosing where you want to be will be slow like a text editor that has loaded too big of a file. IT will be slow at first then jump to where ever you stopped the mouse.
This causes me to have to move the mouse very slowly to find a place I want to build.
Continuing the discussion from Picking starting location very difficult with mouse: