Even in vacuum conditions, light can move slower than its maximum speed depending on the structure of its pulses.
Why is time frozen from light's perspective?
EBC Men's Ministry
Research Shows Faster-Than-Light Warp Speed Is (Probably) Possible
List of consequences of NIL principle, non-interference of light
Fermilab Today
Fermilab Today
Advanced Stellar Propulsion Systems
MindField
Blue Light Sensitivity: Causes, Symptoms and Protection Strategies