Many years ago my mother taught me to rip seams by holding the seam apart and using a razor blade. Fast? You betcha!
When I took sewing classes in school and needed to rip a seam I asked teacher for the razor blade. She darn near fainted!!
Since then I've been using a ripper and cutting every 3-5 stitches then pulling them out with the ripper. Every now and then though if I have a very long seam to rip .... I want my razor blade back!!