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).
NFS@Home is a research project that uses Internet-connected computers to do the lattice sieving step in the Number Field Sieve factorization of large integers.
Java program to factor large numbers. In addition to ECM, it detects algebraic factors for Cunningham numbers and can use the Self Initialising Quadratic Sieve algorithm.
Self-initializing quadratic sieve (SIQS), on a 2GHz Opteron, a 95 digit factorization takes 4 hours, and a 100-digit factorization takes just under 12 hours (possibly more). Public domain library for integer factorization, implementing Pollard-rho, ECM, self-initializing Quadratic Sieve and parts of NFS.
On 2022-06-21, the original page http://www.boo.net/~jasonp/qs.html is off-line: "All Delmarva Online services have been discontinued. December 27, 2014"
With the help of http://web.archive.org/web/20110723030828/http://www.boo.net/~jasonp/qs.html, I could locate the repository https://sourceforge.net/projects/msieve/files/msieve/ which has been updated 2016-11-11. I changed the URL for the bookmark to the sourceforge repository.