4 years ago I set the maximum speed to 20kph i.e. 50hrs. That might seem very low, but the reality was that the fastest time was just under 65 hours and only 4 riders (out of the 60 finishers) were back in less than 70 hours.
(This is not necessarily because it's that hard a ride but because it's set up to ride "collegiate style" with the field regrouping each night. It would undoubtedly be possible for a fast rider to get round significantly faster riding non-stop but on experience of the past editions, those riders prefer to use their extra time in hand to get more sleep. This makes for a more sociable experience on an event with a relatively small field rather than the fast riders disappearing off into the distance - and means resources can be concentrated where/when they provide the most benefit overall rather than having to man controls for hours with very little happening).
This time round based on that experience the maximum speed has been further reduced to 16.7kph (60hrs). That only applies at the finish control - intermediate controls have higher maximum speeds which taper down throughout the event.
The GT version, as noted is set up to be ridden as 4x200km+ @ 14.3kph. There is an overall limit for the finish control as well. The event planner is so heavily geared towards the "standard" event format that there's not really any way to reflect this though.
