You could multiply delta with a var to determine speed of the objects in the scene.
func _process(delta):
var multiplier = 1.0
# 0.5 = slow
# 2.0 = fast
var mov_delta = delta * multiplier
pass
The player would multiply delta by 1.0 while everything else would multiply by 0.5. Maybe create a node group for everything but the player to organize stuff.