I have studied greens in this regard over the years. From my own limited samples (and mostly from my public course designs where cup areas are under 2.25%) 9.5-10 is comfortable for me and all I play with, 11+ is uncomfortable and causes more misses.
Did a master plan for a private club last year, proud of their 13 green speeds every day. They reported five or six greens were scary. I used the old Masters pin location set up method of measuring % of slope both ways at select hole locations, and only placing pins under 5.5 combined. (they prefer 5.0 combined on the front half of the green)
Damn if every tough hole location was at 5.6 or higher, so the system is right on the nose, apparently, at least for country club golfers feeling comfortable on fast greens. BTW, if you use your high school geometry, 5.5 combined slope translates closer to 4% actual slope max) and 3% is the max typically recommended by the USGA.
Of course, borderline scary still translates into more putts, IHMO. Really, if green speeds keep going up, I presume the "casual rules" of golf will emulate putt-putt and require a maximum of three putts before picking up. It's generally what happens anyway in the higher handicap groups, no?