As kVp is increased for greater rates of penetration, x-ray photon velocity:

In the context of x-ray production, the velocity of x-ray photons is constant because it is equal to the speed of light, regardless of the kilovolt peak (kVp) used during the imaging process. While increasing kVp does enhance the penetration ability of x-ray photons and increases their energy, it does not affect their fundamental speed. All electromagnetic radiation, including x-rays, travels at the same speed in a vacuum—approximately (3 \times 10^8) meters per second.

It's important to note that while the energy and the ability to penetrate matter improve with higher kVp, it does not change the inherent velocity of the photons themselves. Therefore, the understanding that x-ray photon velocity remains constant is crucial for grasping the principles of radiation physics and its applications in medical imaging.