Only time consuming factor of the 2.0-2.55 MS I can see is all of the duties/trials you have to do, they're dated content and the queues can be quite long, especially on dps. However devs have already hinted at a way to alleviate this, they mention in a live letter that when 3.0 goes live, you'll be able to do 2.0 duties/trials with parties that are smaller than currently required.

Honostly though, lvling up and doing the story isn't that time consuming in the scope of MMOs. I consider myself fairly casual, and I watched every cutscene, yet it only took me 23 days to beat the 2.0 storyline, and about one day to beat each of the 2.X stories. If someone really wants to rush into HW, I wouldn't be surprised if they could do it in roughly 2 weeks.