Old post, but as I’ve got the same bike, thought I’d chime in:
TBH I’ve found the effort-to-speed quite realistic. I have an outdoor (ebike) with a watt meter (that displays my own contribution) and I know what 100 watts feels like, and I know that on a lighter bike with a decent drive train, with a more aero setup I’d be getting circa 25 kmph on the flat, no problem.
When the zwift app reports that I’m giving about 100 W, it feels about the same as what I get on my ebike.
I think, rather than the “speed” that’s reported on the fitbike computer, look at the calories it reports you’ve used in, say, 30 minutes, for example.
Google for “inchcalculator calorie to watt hour converter”
Note, that the fitbike reports in kcal, so x1000 whatever it reports and input that into the converter. See how many watt hours it gives you.
I put in 40,000 for my half hour ride, (the fitbike computer told me 40) and the converter told me that was 46.5 Watt hours, which is equivalent to a continuous output of 93 Watts for half an hour, which is roughly what I had reported in the zwift app for the ride.
So it’s probably + / - 10%, but for a $400 setup I’m not going to complain. And I’m certainly not going to drop another $300 on pedal sensors.
Maybe they updated the firmware or something, or maybe the build tolerances are so loose that there’s a tonne of variation between bikes, but my experience with it has been fairly OK so far.