want my old (stock) firmware back

i got samsung galaxy sl i9003 3 days ago. it came with froyo (thats all i know about that firmware). it had practically 0 lag and no issues

in a hurry of sorts, i flashed the xxkp7 gingerbread update onto my phone, but it had lag and various other issues, i decided to go back to froyo. after running into brick wall after brick wall, i managed to flash to froyo (jpke1), but the lag is still there (nothing else going wrong though).

what should i do to get the old, lag-free, stock firmwae back?