Andy Kelly's been experiencing the same stuttering, which he's managed to allay only through the use of Battle Encoder Shirasé, a program that limits CPU usage for specific applications. After installing BES and following the instructions in this Reddit thread, Andy says the stuttering has completely disappeared, at least in the last 45 minutes of play. Jarred saw the same success with CPU limiting in DX12 on an i3-8100. If you're running an older CPU, it's definitely worth a shot.
It doesn't take elite programming knowledge or extensive hardware know-how to surmise that something in Red Dead 2's code is juicing CPUs too hard, failing to distribute the processing workload across the cores, or doesn't have the right checks in place to properly limit processes according to whatever CPU is in use.
I imagine it like giving a horse too many apples. A horse is going to keep eating apples if presented them, I know this. And the horse is going to feel energized by the apples and run at a smooth and efficient horse-frames per second, up until the moment its tummy reaches max apple capacity at which point the horse stops, or stutters, in order to vomit up some apples to make room for more apples. A CPU suddenly given too many chunky calculations might choke in a similar way, exhibited by intermittent hitching in-game.
BES and CPU-limiting prevents RDR2 from pushing the CPU to the breaking point, forcing the game processes to chill before knocking your computer's brain on its a*s.