Did you know bike choice in Zwift makes a difference? I've been riding for awhile (876 miles), and only just learned this, so I'm guessing it might be news to others, too!
The idea that what bike you're on make a difference is not a foreign one, and one that I did assume made SOME difference in Zwift, I just didn't realize how much. Given that I recently upgraded from a road bike to my first tri bike IRL (In Real Life), I did the same in Zwift as soon as I "unlocked" one (i.e. earned enough points) figuring a tri bike might be faster in game as well.
Any speed benefit of the upgraded bike in Zwift was quickly negated as I learned "TT" or Time Trial / triathlon bikes are not allowed in most (all?) group rides on Zwift, just as IRL. So I reverted back to one of the other frames and din't think any more of it. I also stayed away from the aero wheels, as I thought they might not be allowed as well.
Fast forward a year, and now that I'm joining Team ODZ for some rides, I need every advantage I can manage to keep up with these fast cats. We'll see if what I just learned about frames and wheel sets makes any difference (fingers crossed!)
The bottom line for newer riders is that you've got one choice—the one my avatar is pictured on above. It's the Zwift Carbon Bike (which every rider starts with in their options), with a Zip 808 wheel set (unlocked at level 10). Somewhat counter-intuitively, you'll continue to unlock other bikes and wheel sets after these that are NOT faster, so there's no advantage to switching until you've progressed far enough to hit level 22 and snag a Canyon Aeroad.
The good news is, the Zip 808 wheel set is as fast as you can (currently) get in game, so early on you'll be able to run on the best/fastest wheels. Working up to the fastest bike—the Tron Bike—is a whole different ballgame. It amazes me how many of them you see in game given you have to complete the Everest Challenge AND climb an additional 41,150 meters to unlock it! Someday...
To recap what's fastest for the beginner on Zwift: Know that the Zwift Carbon is the place to start and stay until you've progressed significantly in game. Keep an eye out for the Zip 808s, and switch to them as soon as you're able. Then it's a matter of working up the ladder another long big to the Canyon Aeroad. For more detail, including test lap data on the fastest equipment in Zwift, see the links below.