Unless it specifically says "the masamune is a weightless blade" in the description of it, the masamune probably weighs the same as a regular no-dachi, which is the type of sword Sephiroth uses. Those things are incredibly unwieldy due to their size, a blade that's taller than a person unbalances really easy, and the weight is no small matter either. It wouldn't be slow, but if used correctly it could be pretty powerful.