If you’re a little clever with interpolation, you don’t need to run at 1000s of frames per second! You’d just calculate how much time after the last frame it would take to cross the line at the last known speed and position.
I always sort of read those jokes as illustrative as GLaDOS being a bad person fielding weak material more than like, an earnest expression of the game writer’s values. Like, the game itself doesn’t present the remarks themselves as funny so much as GLaDOS being rude, snippy, and actively incorrect given that Chell is mega-fit.