When you turn on the High Performance mode, there will be a permanent notification which can't be removed by the usual means.
However, after playing around a bit, I found a solution to hide it for good:
1. Turn on High Performance mode
2. When the notification pops up, long press/swipe it so the snooze option is available
3. Snooze the notification
4. Toggle High Performance mode off and on again
This is what I did and the notification is gone for good and the phone is in high performance mode, so no more clogging up your lock screen or your notification bar
EDIT: Since the June Security Patch, they've allowed you to just swipe away the notification so no need to bother with these steps