* Lightning is a complex phenomenon: It's not like a single bolt traveling at a constant speed. Instead, it involves a series of electrical discharges called "leaders" and "return strokes".
* Leaders are slow: The initial leader, which is a path of ionized air, travels relatively slowly, around a few meters per second.
* Return strokes are fast: The return stroke, which is the bright flash we see, travels much faster, at a significant fraction of the speed of light (around 30% to 50% of the speed of light).
* Zigzagging is a result of ionization: The zigzag path is due to the leader searching for the path of least resistance through the air. This doesn't change the speed of the return stroke, only the path it takes.
So, while lightning's path is often zigzag, it's the speed of the return stroke that's impressive, not the path itself. The return stroke is the part that we see, and it travels at a substantial fraction of the speed of light.
