I think "below .500 in 12 of 19 games" is a bit misleading when taken by itself. That includes games where he either shot > .470 (still pretty good) and/or shot <.500 but had 8+ free throws and/or shot well on 3 pointers.
He's definitely struggling in the finals, and I think it's mostly a result of the "big 3" being actually a "big 1 and an ok 2" and finally hitting another good team.
Look at Wade and Bosh, supposedly the big help for Lebron:
Wade:
vs CHicago: 12.6/3.4/4.8
vs Indy: 15.4/5.1/4.3
Finals: 14.3/1.3/4.3
Bosh:
vs Chicago: 13.6/8.6/2.0
vs Indy: 11.0/4.3/0.7
Finals: 12.3/8.3/3.o
That's essentially playing with George Hill and JJ Hickson from this year as your 2nd and 3rd best players. Tough, and I think he's exhausted from carrying them this far. They (bosh and wade) have the names, but they are waaaay off right now, either they're over the hill or injured, but they've been playing like role players, not 2nd/3rd bananas.
Lebron meanwhile slapped up a 23.6/7.0/7.8 vs chicago and a 29/7.3/5.3 vs Indy to carry them this far. Even against San Antonio, who seems to be the first playoff team to realize that Wade and Bosh are sucking, just focus on Lebron, he's put up a 16.7/12.3/7.3. Any other player, and that would be pretty fantastic. Definitely his shot is not there in the finals, whether it's fatigue or an excellent defensive strategy by the Spurs taking into account his teammates suckitude, but until the finals he shot .438/.353/.808 and .510/.444/.778 against Chicago then Indy, two of teh best defenses in the whole league. (Chi #5, Indy #1, SA #3.) Brutal to be the only contributor vs. those 3 defenses, and in no uncertain terms Wade and Bosh have been AWFUL. But they won't get the press.