At the height of the Cold War, IBM built a computer for the U.S. National Security Agency that was so closely guarded its existence was hidden from the world. IEEE Spectrum reports that the specialized machine served as the NSA's main cryptographic processor, a role that kept it at the center of U.S. code-breaking efforts.
According to the report, the system ran its jobs up to 200 times as fast as any other computer of its time. That dramatic speed advantage came from being designed for a narrow, code-breaking role rather than as a general-purpose machine.
Because this account comes from a single source, there are no competing claims to compare. The report focuses on the machine's secrecy and speed, while the full operational details remain largely outside the public record.