The functions have been implemented for BigInts in v1.4.
Your test case, 15241579244017681, is a worst case scenario for factoring, as the number is only a product of two primes (it has no other divisors).
I put special case code into my implementation of divs to deal with this case. Thus, 15241579244017681 divs will return the correct result almost instantaneously. However, factor and factors don't have that special case treatment yet. They take a really long time to factor this number (maybe an hour!). I guess this is pushing the limits of a mobile implementation.
Other numbers, with more divisors, factor much more quickly, but BigInts don't factor anywhere as quickly as smaller reals.
I'm working on the CAS, and there's going to be a new avenue for factoring huge numbers more quickly.