When it gained energy it would move slower. So, in addition to tardyons and photons, there might exist tachyons. If tachyons exist they can easily be detected by the presence of Cerenkov radiation in a vacuum. Cerenkov radiation is radiation emitted when a charged particle travels through a medium at a speed greater than the velocity of light in the medium. This occurs when the refractive index of the medium is high.
Cerenkov radiation is like the bow wave of a boat, or the shock wave of a supersonic airplane.
Photons bunch up in front of the tachyon and they're radiated away at an angle determined by the speed of the tachyon. Cerenkov detectors are useful in atomic physics for determining the speed of particles moving through a medium. Light slows as it passes through a medium. That's what's responsible for optical effects. There's nothing mysterious about Cerenkov radiation in a medium.