How slowly? I still don't have a buttonhole foot. I do them old-school instead, or I insert a zipper from my stash.
I've been doing more jersey knit baby gowns lately, however, and while it's quite possible to stitch a neat zig-zag without stretching that sensitive T-shirt fabric, it does require some fussing, especially on the neckline.
One of the specialty feet that Grandma Bangle gave me with her sewing machine is a standard walking foot. It works perfectly on all fabrics, but won't accomodate a zig-zag stitch. I kept forgetting that and breaking my needles, so eventually I set aside some etsy profits and purchased an open toe walking foot:
Ooh, it's awesome! I think now that I'll begin to experiment with even fancier stretch-friendly stitches.