Maybe there's a hobby in receiving their trading data and having software react to it — following milliseconds behind their trade.

They’re already 5 moves ahead by the time it hits market.

Milliseconds are an eternity.

...it will be encrypted

How much time does encryption take? I'd wager the fastest links are close to plaintext.

Stream ciphers can operate without adding any extra delay and are considered plenty strong.

At HF bitrates they can use a one-time pad. The only unbreakable cipher.

A single hard drive shipped (or hand-delivered) to each transmitter location ought to last a decade.

No need to ship a hard drive, just send it over the regular internet.

It would be an interesting exercise to design an encryption and compression protocol that has a high bandwidth/high latency link and a low bandwidth/low latency link working together.

> No need to ship a hard drive, just send it over the regular internet.

Well, then it isn't an unbreakable cipher anymore.

It's only as strong as whatever encryption you used to transmit the pad over the clearnet. If you were using a one-time pad to do that then you used up the pad in the process, so you could've just told the reciever to transfer the pad from their internet-decryptor to their HF-radio decryptor.