If I had to guess I'd say that the air probably thermal throttles or reaches a steady-state of lower performance at an earlier time than the Pro, if the pro has a larger cooling package on the same CPU...
If you were to sit the two side by side on a desk and run a complicated pure-cpu x265 1.5 hour ffmpeg encode of a 4K video from raw y4m, the pro will probably race ahead after about 20 minutes, after the heat has soaked into the air.
Yes, that's exactly how it works. I'm just not sure why you're saying this. It's obvious, given the fact that both machines use the same chip and different cooling.
It has nothing to do with the shape and size of the board, which differs between the two machines significantly.
If you were to sit the two side by side on a desk and run a complicated pure-cpu x265 1.5 hour ffmpeg encode of a 4K video from raw y4m, the pro will probably race ahead after about 20 minutes, after the heat has soaked into the air.