Ok, you have me there. I have only one question: why run your own equation to find the magnitude if the square root function runs slower than the magnitude function?
Would it not make sense to use the Magnitude variable if Roblox can calculate that variable faster than it could run the square root function?
Speaking from a game engine design standpoint, Roblox is designed to allow young new developers to learn the basics of game development. So it would make sense for them to optimize the heck out of the .magnitude feature, as it would be more likely for a young developer to grasp the understanding of what that value is, before they learn the math that goes on behind the scenes to find it. Thus I would assume that .magnitude would be lighter on a low end machine than running your own equation.
That is just my two cents though.