This made me want to look at whether the net-rating stuff is particular to these Celtics, or is just "hey the playoffs are hard, all the teams are good and they get better the further you go". So let's compare some anecdata on the best teams since 1990-ish, those with >= 9.0 net rating in the regular season**:
Team -- Net rating Reg Season / Net rating Playoffs (W-L) / Season-to-Playoffs Delta
1990-91 Chicago Bulls* -- +9.5 / +13.2 (15-2) /
+3.7
1991-92 Chicago Bulls* -- +11.0 / +6.9 (15-7) / -4.1
1993-94 Seattle SuperSonics -- +9.5 / +0.9 (2-3!) / -8.6
1995-96 Chicago Bulls* -- +13.4 / +12.1 (15-3) / -1.3
1996-97 Chicago Bulls* -- +12.0 / +6.5 (15-4) / -5.5
1996-97 Utah Jazz -- +9.7 / +3.8 (13-7) / -5.9
1998-99 SA Spurs* -- +9.0 / +9.5 (15-2) /
+0.5
1999-00 LA Lakers* -- +9.1 / +2.6 (15-8) / -6.5
2006-07 SA Spurs* -- +9.3 / +4.5 (16-4) / -4.8
2007-08 Boston Celtics* -- +11.2 / +6.1 (16-10) / -5.1
2008-09 Cleveland Cavaliers -- +10.0 / +9.9 (10-4) / -0.1
2011-12 Chicago Bulls -- +9.1 / -1.9! (2-4) / -11.0
2012-13 OKC Thunder -- +9.8 / +1.4 (5-6) / -8.4
2014-15 GS Warriors* -- +10.2 / +8.2 (16-5) / -2.0
2015-16 SA Spurs -- +11.3 / +9.9 (6-4) / -1.4
2015-16 GS Warriors -- +10.7 / +4.4 (15-9) / -6.3
2016-17 GS Warriors* -- +11.6 / +13.5 (16-1) /
+1.9
2019-20 Milwaukee Bucks -- +9.5 / +1.2 (5-5) / -8.3
2020-21 Utah Jazz -- +9.3 / -0.9! (6-5) / -10.2
Avg Delta, all top NRtg teams (19): -4.4
Avg Delta, Championship Teams (10): -2.3
2023-24 Boston Celtics -- +11.6 / +12.1 (leading playoffs; 7W-2L) /
+0.5
Only 3 of the top 19 teams by net rating have ever
improved upon their regular-season net rating in the playoffs (and all 3 won the title); thus far, we're on track to be the 4th. Many title winning teams have had a negative delta from their regular season, some of them substantially so (but most of those teams still led all playoff teams by net rating). So offhand, I'd say that after 9 playoff games, we're looking damned good, even if the road will get tougher from here on out.
...And either way, we are only 3 years removed from Evan Fournier, Tristan Thompson and Romeo Langford starting multiple playoff games, and I just count my blessings. Long live Wyc's Unlimited Budget (tm).
* Won title
** using Bk-ref's
unadjusted net ratings, because that's apples-to-apples with what they have on the playoff season pages, so it's what I have available.