I just made a king size t-shirt quilt for a boy who works for us. It had some rubberized designs on most of them. I ironed SF101 interfacing on the backs of all the shirts. Did a large meander over the whole thing and stitched on and off the designs with no problem. I stitched at 10 spi and didn't really slow down at all. Maybe it's the fact of the interfacing on the back of the shirts that make a difference. Good luck!