Why does this Motor change speed when polarity is changed
There are a couple of reasons why this might happen.

1) Many DC motors have the brushes arranged for maximum torque and/or speed when the motor is spinning in one particular direction.  Something like ignition timing "Advance" in a gasoline internal combustion engine.

This is done to compensate for the time that it takes the magnetic field to decay as the brushes / commutator breaks the current to one armature winding and moves to the next.

I used to play with this when I was very young - it was easy to change the brush phase angle in some older DC permanent-magnet motors intended for toys.  You would simply adjust the phase angle by rotating the rear plate holding the rear bushing and brush assembly until the motor was spinning at the fastest possible speed.  Then re-crimp the tabs that hold the plate in place.

The motor would always spin faster in one direction compared to the other.

2) (more likely)  The motor that drives the capstan in many older tape decks has an internal centrifugal speed governor.  This is a weight attached to a set of contacts that is part of the armature.  When the motor reaches the correct speed, the contacts open and either reduce or remove power from one or more windings on the armature.

This very simple speed regulator is surprisingly effective.

But: it only regulates speed when the motor is spinning in one particular direction.