clearstream
(He, Him)
I think this claim needs to be taken with a grain of salt, or firmly contextualised. When I am using a published adventure that is replicated on Fantasy Grounds, the encounters (maps, creatures, loot) are all set up. So for published adventures, prep time is less.In addition to the poor UI experience, OP must also understand that using ANY virtual tabletop software will greatly increase DM prep time: it requires you to only to prep the adventure as normal but also all the electronic side of the VTT; depending on the features used this is a serious commitment.
If op anticipates only being virtual for the duration of the pandemic then adopt the most light weight solution possible.
For homebrew adventures (and off-piste additions to published adventures) the prep time could be more because you'll need to set up your encounters.