Yeah, I really need to get of my behind and fix this one :)
So, I am aware of the underlying issue, so you can save up some of your time debugging at least (I am thankful for all bug reports, mind you :) ).
The simplest fix would be to set some default speed limit value when new players join the game, and to introduce data migration that will set a default value for already joined players (if it's not defined yet). I just need to get around to taking care of it (just been a bit lazy/demotivated).
So, as a workaround, for now just make sure to tell everyone to set their speed limit when they join the game (basically what you discovered).