Magnitude checks would not be too costly but It will be inaccurate, if your making melee weapons your best option would be to raycast from a set of attachments on the sword, this module does that: Raycast Hitbox 4.01: For all your melee needs!. Btw the raycasting is not slow, it is only expensive when your casting long rays at a fast rate, however, raycasting a small ray every frame is very inexpensive.