Either ...
... depressing key [X]
or
... (rightclick >) Playback > Playback rate > 1x
doesn't solve it?
P.S.:
Actually [F5] and [F6] result (by default) in an increase resp. decrease of playback speed by 1% and in combination with [Shift] by 10% (so the result isn't necessarily an equivalent of "slow-motion").