One of the things HFT firms take advantage of is increasing a nanosecond lead into a microsecond (or more) lead by route optimization. If they can get information from one exchange to another faster than the original order, they can effectively trade by looking into the future.

Many firms invest heavily in direct microwave links for paths normally served by fiber because of the speed advantage.

Microseconds are not enough, because the arbitrage works by the HFT setting the order in one place, and then rushing the arbitrage to the other place. As long as you're more coordinated than speed of light between the two locations, you're fine.

Also, GPS clocks are cheap and precise, atomic clocks seem gold plating. (Which sometimes is actually necessary in electronics, BTW...)

Hardware (e.g. FPGA) autotraders can have sub-microsecond response times.