I’m assuming this is AI Traffic and not traffic on say the VATSIM network or Multiplayer?
If so, I would say it’s just an AI traffic limitation / bug and believe it’s known (have to search through the forms but believe there was something with altitude and speed).
If you look up those flight numbers on FlightAware you will see their filed speeds are much higher, and the cruise speed (ground) of a 737 is around 800KM/h or 432KTS.