In order to factor (relatively) large semi-primes (i.e. semi-primes larger than 150-bits), you should (probably) just ask YAFU (Yet Another Factoring Utility,
https://github.com/DarkenCode/yafu) to try to factor the number for you in parallel using a highly-optimized number field sieve such as GGNFS (GPL General Number Field Sieve) or SIQS (Self-Initialising Quadratic Sieve). By using YAFU I was able to factor a 302-bit semi-prime in a little over half an hour , as opposed to over the course of 72 minutes using Sage’s Quadratic Sieve (QS).