Can you give the name and location of where the bios files should be on the sd card? root/retroarch/cores/system as SPH5002.bin or something like that? Does it actually make a noticeable performance difference, I did not notice any performance increase after putting the bios thereMost BIOS files are copyrighted, so no download links for you from here.
Though, heres a tip: Just search for psx bios files on google or the like, its rather easy to find them like that.