a simple rule of thumb is that a signal using optical fiber for communication will travel at around 200,000 kilometers per second.

Source: Optical fiber – Wikipedia

I didn’t know fiber was that much slower than vacuum. Apparently this is significant to HFT where they increasingly favor microwave links.

Edit: moar wikipedia: fiber is faster than cat 5e or 10base2 “thin” coax but slower than cat 7 or 10base5 “thick” coax.