When binding off knitwise, as in this photo, the stitch to be passed over the other stitch and off the right-hand needle is picked up in front of the work--easy.
But when binding off purlwise, you have to pick up the same stitch from behind the needle, either blindly, or by rotating the work so you can see what you're doing--either of which slows the process, at least for me.
So here's my trick: Each time I transition from a purl to a knit stitch, I move the yarn from the front to the back of the work before binding off: this allows me to bind off knitwise even if I have just purled. In seed stitch, it goes like this: k, p, move yarn to back, bind off 1st stitch knitwise, k, bind off, p, move yarn to back, bind off knitwise...see? Even though you are binding off in seed stitch pattern, all stitches are bound off knitwise. In 2 X 2 rib, 3 of 4 stitches can be bound off knitwise: k, k, bind off, p, bind off purlwise, p, move yarn to back, bind off knitwise, k, bind off, k, bind off, etc.