Kemper Self-Test (Maintenance Mode)

No I don't think so.

Check if tone come to the output and that all buttons work.

You can do a "init globals" on it to reset the settings to a factory default state. Other than that, you would have to do a manual test on every button and knob yourself.

I think part of the reason it takes so long to boot is some self-checks it does every time it is turned on. This is speculation, of course - I have no idea of how extensive such boot testing is.