I believe there is a kind of level balance that the higher the level of the weapon the better the delay becomes, when the damage upgrades at a certain level the delay is reset and gradually becomes better and this process repeats itself. So the weapons are relatively balanced this way, Rusty Kodachi is the lowest and weakest weapon and should remain so, even with a speed boost it will remain the weakest weapon by 5 damage, but I imagine you're asking for this for Gladiator matches where everyone uses the Rusty Sword as a balanced weapon, don't think I've heard of glads using Rusty Kodachi though.
If the request is for gladiator matches, I'd rather see a training kodachi made the same speed as the rusty, than making the rusty the same speed as the training kodachi. Since the people have been using Rusty weapons as their main gladiator weapon, I imagine these people are used to the speeds and how they work, that is why they choose the rusty weapon, would be a bit weird to change that.
If it isn't for gladiator specific matches, I don't see why it needs change tbh.
