Top speed and acceleration are clearly linked.
Ideally, you want to get up to speed as fast as possible, mindful of the need to brake again later.
Your grip on the floor and the power available from your motors will limit your maximum acceleration. To calculate this properly, you will need to know about the moment of inertia of the wheels, the motor and the mouse as well as the coefficient of friction of your tyres on the maze surface.
The maximum possible acceleration for a mouse that is not being sucked to the floor depends on the coefficient of friction between the tyres and the floor. You are unlikely to exceed 0.5g in a practical mouse and even 0.25g is probably quite reasonable.
Consider: from basic equations of motion starting at rest
v=at
s = 0.5at^2
With an acceleration of 0.25g (approx. 2.5m/s/s), you will be able to reach 1m/s after 0.4seconds over a distance of 200mm or about 1 cell.
This is not too shabby and will certainly do as a goal for now.
Try not to get too despondent over the fact that top-class mice now seem to be able to attain accelerations well in excess of 1g.