How to make faster this code

[Larz60+]

You have a loop within a loop.
That means that each time the outer loops goes through 1 iteration, the inner loop if executed from start to end.
so in your case, the loop:

for i in r:

will run from start to finish from 2 to el for each iteration of list1.
Calculating primes is one place where I would recommend recursion.
see: https://www.daniweb.com/programming/soft...r-function