The only ways to change the party's actual movement speed, short of modifying the executable/Grimrock2.dat/memory, are to give a champion the encumbered or wounded feet status, or to
edit the save game, and these will only give you certain specific speeds.
If you need to move the party extra fast, the PartyComponent:knockback() method could be useful, but it doesn't require the same conditions as normal movement, so naively putting it in the party's onMove hook is not nearly enough.
It is impossible to implement your own party movement that is consistent with the built-in party movement. People claiming that this can be done are either mistaken or lying. You do not have access to the cameraBobbing configuration option, for example.