With the game's self driving assists turned on, I haven't found much difference between tracks, e.g. times to gain 5 levels for the '90 Mazda MX-5 at just below 400 PI were:
Watkins Glen Short 18:39
Homestead Oval 18:19
Lime Rock Full Alt 18:22
If I drive myself, the time for Homestead Oval comes down to 16:28 due to getting all the extra 25s and 50s for good and great segments, and lapping a few seconds faster per lap.