A única maneira que eu sei é isso.
Make sure you are focused on the timeline (click it) and then hit CTRL + A. This will select all editable media on the timeline.
Navigate to visual effects, select clip speed, and drag it on to any piece of media. It will be applied to all pieces of media.
While all clips are selected (VIA CTRL + A) you can easily set the playback speed of all clips in the properties window, under clip speed. This change will be applied to all media.
Before rendering your media, you can set the playback to normal or remove the clip speed from all tracks.
Infelizmente, não é a melhor solução, mas é a única coisa que descobri que funciona.