He's a slow starter, obviously. It makes sense that an offensive player like Federer comes out firing on all cylinders in the first few games while a player like Djokovic usually needs the first few games to get into the groove.
yes it's also right, but it's only a part of the "Djoko-coaster" imo.
A more interesting comparison would be that between Djokovic and Murray. Murray hasn't traditionally been a slow-starter like Djokovic or a full-throttle player like Federer as much as a rollercoasting kind of player who dips and peaks several times during a match. Djokovic OTOH unlike Murray doesn't usually zone out once he shakes off the intial rustiness.
It's not the way I see it : I don't see Murray as much a roller-coaster because Djokovic has this kind of schemes inside one set and even inside one game, for Murray traditionally it's been rather for a whole set, sometimes a whole match, that he goes out of his match. And it looks more that it's about disappointment or loss of confidence whereas for Djokovic it looks like a lack of adrenaline.
And I think that the scheme "shaking off the initial rustiness then going out again" has been traditionally more a Djokovic's scheme than a Murray's scheme.
Although in last years yes, Djokovic has been more like what you say than in the past, and Murray still looks less mature, yes.
It's interesting to note that while Murray is still very much a rollercoasting player, he's often had Federer-like starts this year, coming out firing on all cylinders in the first few games. That's basically how he beat Djokovic a few time this year. It's obviously a new attitude that he's learnt and adapted to with Lendl, and these great starts have helped him cancel out some of the later inevitable dips during a match. I guess that if someone like Murray can learn to be aggressive from the get-go and create a buffer of a few games at the start of a match, then it shouldn't be impossible for Djokovic to learn to do the same. It would definitely have helped him in some crucial moments of his career...
yes you're right : it's possible to do that, and I think Djokovic did that from time to time as well. But it would surely help him to do that in some crucial matches, yes.