The BBC article claims that its advantages over humans were that it didn't make mistakes and would happily churn on for quite a while, with a running time of up to about 80 hours per week.
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 didn't use cards, but paper tape (as seen in the BBC clip). So I guess, in theory, it could have gone on forever if you just glued a fresh tape to the end of the old one.
>"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."
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.