Hello ! Updating my post :  It would be very useful to be able to modify the speed AND the pitch of the session instantly.

The idea would be really interesting for creative purposes, as it is in Logic :

 

The problem with Studio One is that you can already do that with the"timestrech" function in the inspector, but unfortunately you can't do it with instrument tracks or tracks with melodyne on it. You have to bounce it to an audio track first. I would like it to affect the entire session with just a knob. In ableton, there is a device called "MaxForLive" that can do it too.

Thanks