Both passenger and mail varieties of the "Lockhead Vega" and Lockhead Orion" aircraft are not powerful enough to reach their advertised top speed under any condition.
The main selling point of such aircraft is that they are fast but low capacity, perfect for a Very High express service or priority mail. However due to a lack of power they reach nowhere near their advertised top speed and so are only slightly faster than other larger aircraft.
If the numbers are realistic then I guess the problem is caused by bad aircraft physics. Possibly the drag coefficient is too large or the cross-section area of a larger plane is being used. In worst case throwing a gearing on it to boost effective power would work.
Thank you for the report: this was caused by the default air resistance value being too high for such small aircraft. I have now fixed this by lowering the air resistance value for these two aircraft.
The problem also applies slightly to the Boing 247 (both variants). It cannot reach its maximum speed of 304km/h, instead only managing 294-301km/h.
I have just reduced the air resistance value for this, too.