There is the script script_game_get_party_speed_multiplier at which you can determine the multiplier for the speed (see VC modsys for a example on how to use this script, to apply different speeds on land/water, for type of party - lord, bandit, etc -, effect of skills like pathfinder, time of day, etc.) but I don't think there is a way to directly catch the value.
Perhaps you could recalculate the value since the game engine formula for world map speed is known:
https://earendil_ardamire.gitlab.io...Module_System/Module_Parties.html#Party_Speed