The PNG being slow is not due to disk speed. Photoshop is slow to run the compression algorithm for PNG.
Do you need png? TIF will save much faster and will have transparency. The other Pixnub plugins can use TIF images. The only reason you need PNG is if your lab needs it. Most labs can’t run TIF. However, if the cutout images are just being fed into the other Pixnub plugins then TIF works great.