I feel his line start has receded the more time he's spent here - when he first came, he was all about getting out there and meeting his man and proactive in using his size smother as best he could, as quickly as he could. Obviously if he wins the ball, nobody says a word about his lack of pace or recovery on the half-turn and I don't think slow defenders are an issue in high lines if they're intuitive enough to pre-empt the play, and I've generally felt Maguire's not got that awareness about him, like things that should be picked up at source evade him and because of that, fairly benign situations at source massively cascade on his side of the pitch because he's then not only got on overload, but is also out of position.He certainly likes to step up and meet his opponent higher, but he likes to step up from a deeper line. Starting out in a higher line is a very different matter.
As for the main reason of why he gets done so often — it's all of those things. If you have at least one of them (good positioning or recovery pace & agility), you can compensate for the lack of another — like Bobby Moore did for the lack of speed for example (it's a blasphemous example but stylistically Harry at his best tries to do same things that young Moore did — he also loved to step up and cut the attack preemptively even if he needed to go as far forward as central midfield) or someone like, I don't know, Jones, who could've made up after making a positional mistake.
He's too hesitant, and you're as slow as he is, every pause or misstep you can't cover yourself is extremely hazardous.
I'm re-wording some of what you've said, roundabouts, come to think of it! Maguire's lost his abandon since getting here instead of honing what he was into a finer precision instrument, but coaching has a big part to play in that - I'd bet his trajectory would have been different to what it's turned out to be. Overplayed with no guidance or insights into how to improve what he came to the club as or off the back of.