It would have to be at least 25kph since Zwift supports the Kickr Run. For me, it’s not the runners blasting off the front that bother me, it’s the ones that hold the same pace when the route goes uphill.
I’ve actually passed bikes on AdZ with my treadmill matching the incline (I’m a mountain runner living in the flatlands, so I spend a lot of time with my treadmill at 10%+). But yeah, if I’m pushing uphill near the redline and get dropped like a rock, I know that they are probably at 1% or less incline. That is one of the reasons why treadmill races on Zwift will never work, unless it’s on Mayfield, but what’s the point of that? The thing is, it really doesn’t bother me as much on the run, as when I’m on the bike and see obvious “issues”, since most of us that run are just using Zwift to take our minds off the hamster wheel. Although there was one runner holding Olympic 400 meter pace for a 5k, that did get to me.
Perhaps the limit would be 25 km/h, but no one is really running above 25 km/h on a treadmill.
If people are going to be running over 13 km/h in a Zwift race, they should have a heart rate monitor on, and their steps per minute should be over 130 steps per minute, not 101 steps per minute.
I can see some running events that don’t want people in them if they’re running too fast, especially if the event is for slow runners.