The key lock on a player piano can help improve the repetition of
the player action but only if the keys are weighted to be front heavy.

A 'correctly designed' player piano will have keys weighted so that
when the action is operated by the pneumatic the key will stay where
it is.  This means you won't have that annoying 'phantom player' effect
with the keys going up and down.  With this design, a key lock will
have no effect.

Some people like to have the keys moving so some manufacturers weighted
the keys forward of the pivot.  In this case operation of the key lock
will reduce the effective inertia of the action and improve the
repetition.  The effect will be very small and even with moving keys
and no key lock the inertia will be no worse that playing the piano by
hand.

A larger influence on repetition is the design of the pneumatic and its
valve(s).  A late single-valve pneumatic system has a noticeably slower
repetition than an early double-valve design.  My 1904 65-note push-up
will repeat very quickly at low suction levels, whereas my 1920's
88-note inner player (Strauch) requires a higher suction level to be
applied before it will follow suit.  You get what you pay for.

One of my instruments has rear weighted keys _and_ a key lock.  This
can only be to stop someone augmenting the automatic performance.

There are also some other pianos where a key lock would be disastrous.
Many German pianos had the stack under the keyboard and each pneumatic
pushed upwards on the end of the key.  Here, you have to accept the
phantom key effect but repetition is still excellent as all German
pianos I have known have had double-valve action.

Best wishes from GB,
Nicholas Simons