These days, such has chess knowledge progressed, this would be played almost without thought by even intermediate level players. Mind you, Louis Paulsen was notorious for his absent-mindedness. It is possible he was thinking of forcing the d-pawn forward and forgot to play d2-d3 first.

This was a mistake by Paulsen. Much better was 13. Re1 Rxe1 14. Qxe1 Bd7 15. Qf1 Qxf1 16. Kxf1 Re8 17. d3 Bf5

Better was 13... Bd6

Big mistake. Much better was 16. Qa6 Qxa6 17. Rxa6 Rae8 18. Bg4 Bc8 19. Ra1 Rf6

The immediate threat here is 20...Bg2ch 21.Kg1 Bxf3#. What would Black play against 20.Qd3, say? The defensive idea here would be 20.Qd3 Bg2ch 21.Kg1 Bxf3ch 22.Qxg6 hxg6 and White emerges the exchange to the good, though still under a bit of pressure. But Black could try 20.Qd3 Bxf1 21.Qxf1 Re6 22.Kg2 Re1 23.Qxe1 Rxe1 24.Ba3 Re2 with a (probably) winning end game. Unfortunately 23.Ba3 looks to be an improvement over that line. But Black also can try a different line. I think I can see why Morphy took the full 12 minutes: 20.Qd3 Bxf2, threatening 21...Bg2#. But White still has 21.Qxg6 hxg6 22.Rd1 Re2 23.Ra5 Bh4 24.Rg1 Bf2 25.Rd1 and a draw by repetition. Black's best move looks obvious, once you think of it: 20.Qd3 f5! There would follow 21.Rd1 Bg2ch 22.Kg1 Bxf3ch 23.Kf1 Bxd1, with the deadly threat 24.Ra1 (say) Be2ch 25.Qxe2 Rg1ch!! -(a vital finesse!)- 26.Kxg1 Rxe2 and Black will win with his K-side pawn majority.

This blunder by white gave black forced mate in at most six moves. Much much better was 20. Qd3 f5 21. Rd1 Bg2+ 22. Kg1 Bxf3+ 23. Kf1 Bxd1 24. Ra1 Rge6 25. Kg2 Be2 26. Qxf5 Rf6 27. Qxf6 gxf6

An inaccuracy from Morphy. Bg4 & black has triumphed!!

23...Bh3+ blows the forced mate. A better continuation would have been 23...Be4+ 24. Kf1 Bf5 25. Qe2 Bh3+ 26. Ke1 Rg1#

The big blunder in this sequence was 28 d4. Better for white was 28. Kg2 Bb6+ 29. Kg3 Rexh2 30. Kf4 R2h4+ 31. Kf3 Rh3+ 32. Ke2 R6h4 33. Kd1 h5 34. Kc2 Rg4

I don't believe other players of Morphy's time were ignorant of the need for quick development, exactly. I suspect that Paul Morphy's advance on his comtemporaries might have have been more in the nature of his appreciation of time and tempo, a far more sophisticated and far-reaching notion. Development is only part of this: the importance of time stretches into the end game. There always seems to be a fast pace to Morphy's games. But remember this, although he beat Adolf Anderssen by a large margin in their match, Anderssen won the first game - a closed positional struggle in which Morphy was much less 'at home.'