I understand that this game has a very demanding computational load, and that's not a problem, but what is surprising is the high CPU usage when the game is paused with the options menu open. Indeed, even when the game is minimized and doing nothing, it maintains a very high CPU load.
To replicate this problem, I start the program, and open any save-file. With no in-game time elapsed and without acknowledging the starting message, I minimize the game and observe the CPU usage using the activity monitor. ONI's CPU usage remains pinned at 85% indefinitely. See the first image attached. So, the physics engine has been loaded, but should not be running. It's the kind of CPU load I'd expect from an emulator.
As a comparison, when the game is running at the double fast-forward speed, the CPU usage is 170%. See 2nd image attached.
I'm running a 2017 Macbook Pro, 2.3 GHz Intel Core i5, 16GB 2133 MHz LPDDR3. The game was installed via Steam.
start the program, open any save-file, leave the game paused.