I'm sure your prep time will continue to fall as you get more accustomed to the new edition.
For the longest time, I wouldn't prep D&D and many other games, at all. Stats were a pain to pull together and write down, and would often end up needing to be adjusted on the fly, anyway, so I got used to skipping the prep and ballparking/adjusting the stats as I ran the game. Works great for sandboxing, obviously. That probably reach a crescendo with 3e, which was so involved and intricate that I only ran one relatively short (if successful) campaign in 3.0 before deciding to just play - a decision that felt particularly right as the community shifted more and more towards reverence for RAW (my one 3.0 campaign used a number of rules variants that, I'm guessing, would not have been easily accepted in 3.5). 4e I started running with Encounters and had a published adventure to work with, not something I usually did, but I found I didn't need to modify it on the fly like I was accustomed to doing, so, when I started running an original 4e campaign, I started doing the prep work for it, which, between encounter guidelines that gave fairly consistent results and the monster builder tool, was surprisingly easy.
With 5e, I find I'm back to my more accustomed style of adjusting combats on the fly when I run from a module, or just making them up as I go when the players jump the rail and I decide to throw something at them. I did try running the first few sessions of HotDQ 'by the book,' and (since I was going to run two of them again at a convention) also tried using the encounter guidelines to 'fix' them ahead of time. Neither worked out well, the encounter guidelines are more complicated than they need to be, and don't deliver consistent results. It's easier, and, I think, gives better results, to just have a basic idea of a possible encounter (goblins hiding in the rocks; skeletons emerge from niches in the wall; you walk into a gelatinous cube; whatever) and iron out the details as it progresses. Running TotM, it's even easy to adjust the number of monsters in an encounter on the fly, since the players don't have the luxury of just counting the minis you put out, and you're under no obligation to use precise numbers in your descriptions. Seems to be working well - when I get run.