You could just keep slow as the default and make it a thing - like, when a Magical Boy/Girl begins their fancy transformation the sudden influx of magical energy bubbles around them creating a sort of impenetrable barrier that lasts until the transformation finishes? Or something?
I just have this image of some guy getting hit by a big blast and then, when the smoke clears, he stands triumphant, completely unscathed… and wearing a nice frilly sailor scout outfit. 