I strongly agree with @tobiaseigen on this because, even when I searched this full ‘Meta’ site (with or without Ai) as to what is this ‘Mini Profiler’ is and what does it do, I couldn’t get any idea. My search, at best, just showed me this very post, to which I’m replying.
And at the risk of sounding a bit dumb, even @tobiaseigen’s screenshot showing ‘what the mini profiler was’, I judged it wrong and thought that 'Sascha Hofmann’s profile was what was being referred to as the ‘Mini-Profiler’. Because, as I thought, the black popup (shown again by me here below) showed the ‘Profile’ of ‘Sascha HOfmann’ (small, white popup, 345.2 ms was very easy to be ignored, even by the red pointer 
).
And besides the absence of any documentation or help on this mini profiler, I do believe that this MP should On by default.
Because, if there was no ‘Mini-Profiler’ on by default, I couldn’t have judged that there is one, even after going thru all this, had it been not me looking at this screen shot again and again, to be very sure, I couldn’t have discovered it (and its utility).