It probably won't work great. The Starlink unit has a phased array forming narrow beams of reception and transmission to the satellite, and assumes it's on a stable platform.
It's possible to build a system that is robust to bounces and yaws, but that doesn't mean that they have.
They are already selling commercial service for airlines.
So yeah, it'll work at the speed of a van.
https://www.tesmanian.com/blogs/tesmanian-blog/starlink-x
https://www.cnbc.com/2022/04/25/spacex-signs-hawaiian-airlin...
Actual recordings in motion of normal end-user terminal in motion. It works OK-ish. But a lot of packet loss (completely lost packets) and latency spikes (from retransmits).
Of course, with ground hardware built for in-motion, you could do significantly better.
Source: https://www.tuckstruck.net/truck-and-kit/geekery/starlink-mo...
I'm guessing utilization is rather high with retransmits, etc-- which is probably a driving factor of Starlink prohibiting in-motion use in EULA.
Note it's not really the speed that matters. It's suddenly having a much different bearing to the satellite because you have begun a turn or hit a dip in the road that matters.