[Geoff Bacon]

You are always going to have an issue with users not understanding what "Performance mode is enabled" means as you can't force them to read the manual!

When enabled, how about a faint transparent orange "performance mode" icon at extreme bottom right corner ?; this is unlikely to obscure anything (beware possible clock position!).
Hopefully, after many presses, the user will notice it and "wonder what it does".

I'm not sure what you do when it is tapped e.g. turn off immediately, display a dialog or display the normal selection mechanism (can't remember what you call it).
I suspect turning performance mode off immediately is probably easiest to code and simplest to understand (with a disappearing "Performance mode disabled (editing now allowed)" message so users realize what it did). MSP competent users will know how to re-enable performance mode should they inadvertently tap it.

If necessary, this icon could be suppressed by a setting (but I'm not a fan of large numbers of options)

No rush - it's been like this for a long time so a bit longer isn't an issue.

Geoff