Yup, a double 'for loop' in python isn't efficient enough. It works fine on the tests but times out when I attempt. Funnily enough, I was able to pass everything by doing a very simple approximation (which you're definitely not allowed to do) so I guess the tests need to be more strict..
Edit.. a double loop is actually enough lol, you just need to do the summation directly. Now I get why everyone was saying this is not 4 kyu