With observer effect, a photon it will go at max speed.
Without observer effect, photons will cause "shockwave effect" because it will be pushed by rotation of planets, speed of interstellar object, galaxy rotation and movement, etc.
Shockwave effect causes the fabric of space and time to be messed up, allowing a multitude of beginnings, thus the photon will cascade and make wave effect, since there are no set reference of space and time, because lack of observer effect.
The shockwave effect explains Path Integral, Dark Matter, Quantum Eraser, etc.