Isn't this kata too easy for 5 kyu? Or is it just for Python (where the solution takes one line), and it's much harder in other languages?

Nice, but is it worth 4 kyu? :) This kata is very straightforward, and you almost don't need to think, just implement what's written in the description.

dfhwze's comment exhaustingly answers this question.

• There is no essential factorisation technique required. Several different techniques have been used to solve this kata.
• Factorisation definately isn't the only bottleneck to solve this kata ;-)

Can anyone tell me if implementing specific factorization algorithm like Quadratic sieve is essential for this kata? I have a feeling that factorization is the bottleneck, but maybe I'm mistaken.

(I'm not sure if this question is a spoiler, as it's not that far from what is written in the article linked in the kata's description :)