From mprisV2.2 documentation
http://specifications.freedesktop.org/mpris-spec/latest/Player_Interface.html#Simple-Type:Playback_Rate
mpris2.types.playback_rate.
Playback_Rate
Bases: float
float
A playback rate
This is a multiplier, so a value of 0.5 indicates that playback is happening at half speed, while 1.5 means that 1.5 seconds of ‘track time’ is consumed every second.