Converting string into number does not work properly [closed]

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Sage can convert huge integer number into string very quickly but the opposite way it is very slow:

n=2^7000000; s=str(n); (very fast) m=int(s); (very slow)

I also used "eval" and "sage_eval" instead of "int" with same result, i.e. converting from huge string into integer number is very slow. It should take about same time in both cases since variables n, m and s are of the same length.

Do you have any idea why is this so? Mathematica is much more faster in evaluating the above operations.