I don't know how large the numbers were that needed 10 seconds to multiply, but even if it is not much faster (or even if it is slower) than a human for a single operation, a human won't be able to reliably do arithmetic over any comparable time frame.
>"It was, of course, slow, not much faster than hand calculation on single operations, but fully automatic, extremely reliable and utterly relentless. It took little power and could be left unattended for long periods; I think the record was over one Christmas-New Year holiday when it was all by itself, with miles of input data on punched tape to keep it happy, for at least ten days and was still ticking away when we came back."
http://www.chilton-computing.org.uk/acl/literature/reports/p...