I think it’s because the game tries to execute as fast as it can, there’s no frame-limiter.
So it wants to do a lot of executions each turn so your OS assigns it more processing time.
Not sure what can be done about it, but the game in many situations loops through a helluva lot of objects to find parts and find items and references to those items and so on and so forth.
It could be made much more efficient, but it’ll take a lot of work to do so.