Can you type something in the number field next to the slider?
Set the physical mouse to a higher dpi? Probably not what you’re asking though.
If the maximum speed pointer is too slow (which can also be subjective) for your touchpad, this might a be driver bug or some missing calibration for your variant of hardware. Reach out to libinput devs, they track issues here: https://gitlab.freedesktop.org/libinput/libinput/-/issues
For me putting the slider to 1.0 makes the touchpad so fast it’s barely usable