The tempo allows a composer to engrave the speed at which a score section or a whole score needs to be interpreted. Score sections can be played at different tempos. Thus, users checking a score without writing rights will not be able to change the tempo.
Playback speed, however, is not engraved and will not be persisted into the score, allowing every user to change it for every score. Furthermore, it changes the global speed, acting as a speed multiplier to whichever tempo is currently being used.
You can change the playback speed in the quick playback options, by clicking on the down arrow on the right of the play button. Here you will be able to accelerate or slow down the global speed with the + and - buttons.
There is currently a delay of no more than 2 seconds after the playback speed has been changed while the score is already being played back. This is something which we aim to improve in the future.