In practice, a live performance computer can sometimes be used both for the performance and for other tasks on the road.
When intending to use a non-dedicated computer on stage, we always make a separate bootable partition, called something like STAGE.
On this partition, we make sure to start with a clean install of macOS and then install only the software needed for the performance. On this partition, we apply system settings suitable for performance.
When preparing to go on stage, we go to System Preferences > Startup Disk and select the STAGE disk as the startup disk. This makes it easier to quickly restart the system in the rare case something irrecoverable happens shortly before we go live.
Switching to another partition is done in this panel too. Alternatively, you can restart the computer and press and hold the Option (⌥) key on your keyboard when it starts booting up.
We recommend using this partition exclusively for the live performance. When the performance is over, switch back to the normal disk. We test the stage setup thoroughly and then never change it or install any software update on the STAGE partition unless there is ample time to test the new setup.