I, too, see this as classic nitrogen narcosis. Forget those stories about happy drunk and donating air to the nearest fish, IME the classic narc is just as you describe: Molasses for brains and mental tunnel vision (lack of ability to concentrate on more than one task at the time).
Some people are more susceptible to narc. IDK if that's because there's a real difference in susceptibility, or if it's because some people are better at noticing it, or if it's a combination of the two. I can often feel the onset of narc somewhere between 25m and 30m, but normally I don't think it's impacting me significantly at that depth. But the response varies a lot. I've been (IMO, of course) completely clear-headed at 30+, and I've been thinking a bit sluggishly and/or felt the whiffs of dark narc shallower than 30m. So, being of the careful type, I've set my personal depth limit at 30m to maintain a decently clear head. Besides, for a single tank rec diver, deeper dives than that would become very short, so I don't really see much point in going deeper. IMO, of course.
When I did my AOW deep dive, I wasn't noticeably impaired on the math test, but on the letters test, I could notice my brain working somewhat slower. I'm a math person, not a language person, so the math was more hard-wired into my brain and less affected by the narc. Any new, unfamiliar task will be more difficult when you're DUI (diving under the influence), while stuff you've done hundreds of times are more easily managed. At least until you have a situation, then it might more easily escalate to an emergency. Which is why doing drills often is a good thing, you move the exercise of those drills from the conscious part of your brain to the unconscious part.
I'm not particularly surprised that things improved for you by an ascent from 30m to 27m; I've had similar experiences. IME narc can hit suddenly, and it can go away quite suddenly by a moderate adjustment of the depth.
I've heard a guide just blow off any suggestion of narc at 29m, because "you don't get drunk shallower than 30m". Even though it was a clear case of molasses for brains, just as you describe it. That guide didn't understand that narc is a continuum, and that there are both personal differences and differences from day to day, from one set of conditions to the other.