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Well the mining instructions say not to use the same key on more than one witness so each probably has a separate key to begin with. Just a matter of sweeping the accounts as soon as possible.

It could just be someone has installed in on every computer in some university labs or something like that too.

Each node is finding a POW in 2-3 hours. You can't do that with PC using the standard miner.

Agreed. Most likely he created a GPU miner.