ok sure, I changed a little bit the code ( I don't know why I make this stupid error ), defining dx as a fix number, so now it is a bit faster, but not so much. :)
And I add a function to calculate the time.

Now I arrived to the 7nt decimal digit correct. (below the approximation there is the time in seconds)

And the "graph" is:

x axis : correct digit
y axis : time

Obviously it has an exponential shape, and for sure Python is not indicate to make calculations.