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.
Hmm another one of these threads about twighlight princess we quite literally get one or two a month on here about the load times does nobody ever check to see if their question has already been asked?? Anyway on a side note it loads in about 6 secs for me using my Sandisk class 10 32gb card anyway why does everyone complain about the load times I had a zx spectrum back in the 80s and games took 5 mins to load then you would finish a level then need to wait another 5 mins for next level to load people today are just far too impatient.