I agree that if you are spending time putting set pieces together you are going to spend a lot more time prepping. That said, preparing set pieces is antithetical to improv IMO. I'm not saying it is a bad way to play or anything, just that if you are essentially locking the PCs into set piece, you are going to guide them to it rather than just react to what they are doing.
I do think that it takes more prep time to run a prewritten adventure than to create your own or improv. You not only have to mine the paid-per-word prose for obfuscated important information, you have to personalize to your group. I never used to run prewritten adventures until I started running on Fantasy Grounds, because I found improv on the VTT very difficult.
I run a very improv heavy game myself, just pointing out that different people have different styles. But sheesh, VTT is so much work to get right. I got it all to work and it was kind of cool when everything came together but I'd want a massive library of set pieces to really do it right, even though I do a lot of ToTM for non-combat encounters.
My ideal VTT would have an AI of some sort help you make your scenarios. Something like "I want a large tavern, populate it with this mix and it's not very busy" or "give me an abandoned mansion in this style, it has X number of rooms". But that, at least for now, is still a pipe dream.