It’s about 200,000km/h when traveling through fiber optic cable.

I think you meant kilometres per second, not per hour.

Very simplified: the speed of light isn't constant. The well-known 299.792.458 m/s constant is the speed of light in vacuum - and glass isn't a vacuum. Light goes significantly slower in a lot of mediums, including glass, and it's why things like lenses are possible.

It is also why high speed trading firms invest in microwave radio links the speed of light through air is enough faster enough than the speed of light through glass that they feel this gives them a trading edge.

Honestly, gaming the system this hard really worries me, a lot of our economic ability is tied up in these trading system(the stock market). and I can see something going wrong far faster than our ability to fix it.

Flash crash. It's really hard to put in fair minimum times on things. A basic delay isn't really any better.

Speed of light in a medium is c/index of refraction, which is about 1.5 for every glass and highly transparent plastic.