One of the first electronic computers, the Colossus Mk2, has been rebuilt using vintage parts. It was a code breaking machine, used to decipher German messages and provide critical intelligence during the late part of World War II. From the article:
He [Mr. Sale] argues that the original Colossus was so powerful, it would take current computers the same amount of time to break codes.
The machines were so successful that by the end of the war, 63 million characters of German messages had been decrypted.
I don’t think I buy the first bit; modern computers are quite a bit faster than the original ENIAC and other computers of that era, but hey, what do I know.