How do i make an OnEnter walkspeed script?

Here's how you would normally do it..

game.Players.PlayerAdded:connect(function(player)
	player.CharacterAdded:connect(function(character)
		local hum = character:WaitForChild("Humanoid")
		hum.WalkSpeed = 16
	end)
end)

But in your case, since something else is changing the walkspeed, you might want to wait before changing it, like this.

game.Players.PlayerAdded:connect(function(player)
	player.CharacterAdded:connect(function(character)
		wait(.1)
		local hum = character:WaitForChild("Humanoid")
		hum.WalkSpeed = 16
	end)
end)

Again, you should probably fix the thing causing it. I just posted this in case other people want to see it.