Limit your responses to a few paragraphs at a time. Trust me, it's not as hard as it sounds. Once you get past the part where the RP's opening up, barring a few exceptions, you'll be able to get away with smaller posts. When I gm, most of my posts are the smallest - even when I'm taking a direct hand in things. It gives the RPers more freedom, I feel. That all said, yeah, you'll have to spend about 20 minutes per response if you want it meaty.
Don't necessarily wait 'til everyone's posted, respond to those posts you can when you find the time.
EDIT: Remember that in RPs, time is fluid. Maybe not everyone acts at the same times, or even the same block of time. If someone looks like they're getting ahead, it'll turn out they were actually starting a day or two before everyone else, or maybe the others had a few days of downtime. Unless everyone's in the same space at the same time - ie, in the same city near enough to interact - then they can be treated as being in their own timeblock, which can be shuffled about or even extended to fit in with the timeblocks of everyone else's actions.