Why does frame rate matter for speedruns in games with in-game timers?

Question

Watching this video from Karl Jobst, he mentions that the speedruns of several Doom ports must take into account the game's poor frame rate.  I know the 007 Goldeneye speedrun on the N64 also involves keeping the frame rate as high as possible.  I assume many other games have similar problems.
But I don't understand why frame rate matters in these two games, because they use in-game timers for their timing.  I would think if the frame rate dipped, it would slow the timer down just as much.  What's going on here?

hobbs · Accepted Answer

I would think if the frame rate dipped, it would slow the timer down just as much.

That's where you're mistaken. The in-game timer isn't a frame counter, it's a timer. Either it's driven by a timer interrupt triggered by some clock in the system (probably divided down from the same clock that runs the CPU), or it's driven by a vblank interrupt that triggers at the display framerate (which is generally a fixed commodity on consoles). Either way, it's mostly independent of what the rest of the code is doing. If the game engine fails to deliver a frame on time, it results in a gameplay slowdown, but it doesn't result in less time being counted.
Of course not all games operate this way, but the ones in question do, and it's a common enough approach.

Why does frame rate matter for speedruns in games with in-game timers?

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