We would recommend using actions to adjust the scrolling speed when performing live.
To do this, open the Utilities Menu by tapping on the gear icon in the Menubar. Then choose Editors and Actions and choose the Screen Tab on the bottom. This will show you a graphic of the device's screen. Tap on a hot corner to assign an action. Choose the Autoscroll > Speed > Increase/Decrease action. You can then assign the other Increase/Decrease action to another corner. Tap Done to save your changes.
Now when the song is running, you can tap on a hot corner of the screen to increase or decrease the speed.