[wizardishungry]

Generally, you're only required to provide changes if you are distributing your modified program. This may have been partially addressed in GPLv3 in the form of an "application service provider" clause, but I don't recall

[azernik]

I doubt they even fell under the "application service provider" clause; speed-trading software is usually used completely internally by a financial firm to make its own trades.

[steveklabnik]

IANAL, but I think that's the AGPL, not GPLv3.