Using the small curved blade seam ripper will get it done fast. Separate the layers and cut the threads as they open up when you pull on both layers. So it's one hand on the top layer, one hand on the bottom layer and one hand on the seam ripper. That's three hands, right? You have three hands! I put the bottom layer under the presser foot and put the needle down just so that the fabric is caught. Gently tug on the top layer as you cut downward with that curved blade. The blade will cut the third or fourth thread down as the fabric separates.
I gave a demo at our quilt group on how to do this, but didn't have my sewing machine handy. I had a C-clamp, so clamped the bottom layer to the table edge for my third hand. I could undo a WOF seam in just a few minutes. (Yes, I sewed way too many of my strips together when they should have been added in a "quilt-as-you-go" method.) Didn't take me long to undo them all.