As FI approaches, I've also become more checked out at work, been working fewer hours, more relaxed, got promoted, feel a lot less pressure, but respect/reputation/visibility grew (IMO), which may be because I delegate more and only focus on the truly important stuff (I do feel my productivity per time unit has improved), and/or because I feel more healthy/confident psychologically and that's all people care about :) In any case, things are getting more fun -- no stress, easy, got a raise to ~$320k total comp now, which makes FIRE almost obsolete now, paradoxically.
It's funny, this whole thing reminds me of married couples who worked on their marriage for years, got counseling, tried to fix problems, etc. and when they finally decide to divorce and move out, they start rekindling again, going on dates, and things start to go well again. Psychologically, this is probably the difference between seeing the glass half empty (and whining about problems) vs half full (being appreciative). In relationships though, starting over probably won't work for the same reasons it didn't work before, but with FI, it seems you're getting a lasting change in your perception.