There's a hardware limit on how fast the Wii U can read data off SD cards. You can try reformatting your SD card with 64 KB clusters if you haven't done so already in order to speed it up a little bit, but you're going to have some issues with the loading times no matter what you do. Until we get access to something like USB-loading with IOSU, you're stuck using SD cards on Loadiine.
Or you can buy the game.