I think it's unlikely that SpaceX does any speed throttling, because it would run counter to the objectives of a beta test.
But what they are testing for sure are different encoding, encryption, compression and forward error correction algorithms. To find the right balance between raw speed and not loosing any packets because of things like rain and snow.
Another challenge all for itself is how to distribute the available time slots for data transfer between all active terminals within a cell.
Even if there is no hard data cap, many satellite internet companies will throttle download speeds if you pass the allotted amount of data. This is the norm for satellite internet. But Starlink is changing a lot of the norms of satellite internet, and that includes providing unlimited data.
