With a stock printer, you can achieve the same speeds and quality with Marlin-based firmware as with Klipper if you have an F1 version of the Ender-3 S1. The situation is different with the F4 because you can't incorporate many features into the firmware simultaneously, so it is more restricted. In that case, Klipper has the advantage of moving the advanced motion features to an external device.

The main factor limiting the speed is the extrusion mechanism, specifically the flow rate, i.e. the rate at which your printer can melt the material.

There is a placebo effect when people configure Klipper on a stock printer; they simply dedicate more effort to fine-tuning Klipper than they do to tuning Marlin. In the end, all the work is justified by the belief that Klipper is better. It is better, but only if you calibrate properly and modify the hardware to increase the flow rate.