this question is intended to be digital platform agnostic unless it
has significant bearing on the answer.
Unfortunately, It does. In almost every way. Because each tool you choose will be chosen to replace one or more of the functions you otherwise have in real life. And we all role-play just a little differently than each other.
Theatre of the Mind
How 'Theatre of the Mind' are your games? Do you play with no miniatures or maps at all? Do you pull out the minis just when things get thick? Do you map and mini even the tavern scenes no matter how non-combat role-play heavy they may be?
If you don't need minis on maps... you honestly don't need a VTT (Virtual Table Top). You'd be fine with Discord, you can send "hand-outs" to the group, including maps. You will almost definitely want to use a voice chat channel (but not necessarily, you could type everything).
If your players are experienced, you can even describe the combat and handle the minis yourself, and only once-in-awhile fire a picture of the battle situation to them.
In those above cases... your prep time almost doesn't change. As with all options... YOU need to explore the tools and understand them, but once you're comfortable it should feel basically the same come game time. As the game will play out virtually the same, your prep time will also be virtually the same. Perhaps a few minutes for you to 'digitize' your otherwise physical materials.
Minis and Grids
If you use minis and grids... you're going to want a VTT. You can do it other ways, but you will be fighting an uphill battle.
As far as how this effects your prep-time for games; Once you're familiar with whatever tool you're using, it goes rather quickly. If you want a number... maybe 10% more time importing assets and setting permissions for what player's get to see, what they get to see later on, and what they'll never get to see. These tools have been designed by gamers and they know what we need and don't need in our way.
Your instincts are correct, though. It's your preference for winging-it on the encounters that could be an issue. Your players will either need something to do while you set up something from scratch, or YOU need to pre-spend that time by having a few environments and maybe a few stacks of baddies ready to go. If you do that, you'll be putting in literally minutes extra on prep... but you will save each and every one of your players the same number of minutes waiting.
But, these VTTs usually do several things your in-person table doesn't do. Like handling line-of-sight, vision angles and distance accounting for lighting, walls and elevation. These additional perks are NOT mandatory. If you don't use them, you don't add any time to your prep. If you want to use them (and they're pretty cool) it will cost you prep-time. Again, it's hard to say how much because you need to play with them and decide how much you want of each of those things. But I'll say, some basic perks can cost you an hour before a game. In depth fog-of-war, actual walls baked into your maps, invisible traps waiting to be flagged as visible (and painful)... you could spend several hours building a dungeon for your players.
The fact is, though, some time is going to be 'lost' while you familiarize yourself with the platform. That's unavoidable. And some time will be spent telling your players how to use their ends of the tools. And your patience will likely be tested when you have to tell them for the 18th time what they need to do to roll a Dex Save with Advantage. But these things will pass, and it will become comfortable for everyone, and it will just be the new table you play your RPGs around.
I know, you didn't ask, but someone will. What tools should I use?
There's arguments for all of them... I'm not gonna get into that.
Start with Discord for your voice and text chat.
If you need a VTT: (can also replace Discord for chats, if you'd like)
- Roll20 is the gorilla (requires subscriptions if you really get into it at all)
- FoundryVTT is the best (does everything, friendly single-person indie developer)
If you need character and material management:
- DnDBeyond (official source of digital material, excellent character management)
- Beyond20 (chromium/firefox extension bridges DnDBeyond characters into VTT, amazing)
If you're using Discord, you'll likely want a dice roller:
- Using the Beyond20 plugin above... already can send rolls from DnDBeyond to Discord.
- Sidekick Is the dice roller you'll want to add to Discord, otherwise.