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This point can also be addressed in two ways: empirically and theoretically. Empirically, botnets so far have not been substantially involved in mounting 51% attacks in existing altcoins, generally, they are content simply to extract revenue as "legitimate" miners. Theoretically, the current public prominence of botnets as a threat is backed largely by their performance in network flooding / denial of service attacks - situations where each computer in a botnet is only used for the number of connections that it can make. In the context of mining, however, botnet-infected computers tend to be running older operating systems and have weaker specifications, meaning that they will have a much lower performance and impact on the network than their size might initially suggest.
How long will it take to build such a network with enough nodes all over the planet to handle the traffic?