Very simple, maybe a little too much. It's true that not being able to do anything except "New Speed" is a little boring.
But my monkey brain still like the car going faster on my second screen. 100% honest, I'm a simple man, if there was multiple cars, different upgrades, or even just another track it would have been a great simple experience.
And while writing my comment I saw that the car changes colour when going faster, or just changes over time, but still a very nice subtle thing. And I guess the game not being infinite made my brain sad.