The data rate you get depends on three things:
1. The quality of the radio link to the eNodeB (4G Base Station). In an airport lounge it will be in the ceiling somewhere, so quality is excellent
2. Who else is using it and how good their links are (you are all sharing the same resource, so other users reduce what’s left for you and if they have poor radio links they will take out even more capacity as it takes longer to send the same data to someone on a worse quality link)
3. The bandwidth rules applied by your operator eg they may start capping everyone to a maximum instantaneous connection speed even if the whole network is empty. Sounds perverse, but they often do this so you don’t get disappointed by only x Mb/s as you had 2x the week before.