For many practical purposes, the speed of light (hereafter I’ll call it “C”) is “infinitely fast”. If you define infinitely fast as the speed you’ll be going if you accelerate forever, then C is exactly that. At a basic level, speed is just distance traveled over time taken (as in “miles per hour”). Also, consider this: at any other speed you can speed up or slow down, but at C you genuinely don’t have time to step on the brakes or the gas. When a photon is generated it immediately takes off at C, and never slows down until it runs into something.