If you give someone a speed of 250kts and he is 10000' feet higher than the preceding aircraft at 300kts, the actual difference in True Airspeed is only about 20kts. In other words, IF the winds are exactly the same, then the difference in groundspeed, or what you see on radar is a lot less than what you think it should be. Factor in a higher windspeed, which is possibly a tailwind, and now the higher aircraft may in fact be overtaking the lower aircraft in spite of the fact that his Indicated airspeed is 50kts lower.
I believe there are 2 general rules for Indicated to True airspeed, one is that for every 1000 foot difference in altitude there is an additional 1.5% added to IAS to get TAS. The other one is that for every 1000 feet altitude difference add roughly 3kts to go from IAS to TAS.
As an example 2 different aircraft, one at 10K' and the other at 20K', showing exactly the same IAS, and in a NO WIND situation, the higher aircraft would be doing approximately 30kts faster groundspeed.