why is this position +4.5 for white?

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4RodrigoSalazar
This is from a Queens gambit tutorial on Youtube where black makes a mistake and ends up here. Unfortunately the tutorial stops right here and the speaker says white has +4.5 without explanation on what's so great about the position.

When I place it into Chess.som and see what moves the computers would make against each other it's not really clear at all that white has an large advantage.
 
 
black to move

 

daxypoo
black king is permanetly in center; knight on g8 hanging; and black dark square bishop can show up quickly and hit from the other side; note has nothing to clog up the works on the 5 or 6th ranks

would you rather be white or black here?
jonnin

the answer is what the position is after the next best few moves.  guessing at some moves (its set up wrong thinking white to move here, so computer plays bxN at 7+ point lead) without redoing it that its Nf6 exd Qxd? Nf3 Q where... e4+?  Be3 Nc6 O-O and now what?  White has castled and developed, black has moved his queen twice and lost a pawn and bishops are not only not developed but can't do so.   I guess black could try b6 and Bb7 instead, but its still off to a terrible start. 

White has a tangible 1 pawn lead, and if black can get back into the game the positional lead will vanish.  White will push hard to prevent that.