You're correct, the slowdown is related to the GPU. You can see this easily if you look in a process viewer; you'll see that the GPU thread is almost maxed out at 100%. IIRC, it has to do with the minimap which is insanely intensive in Hyrule Field.
Normally the only side-effect the speed hack has is that the minimap has weird borders, but other than that, it shouldn't cause any other issues.
Normally the only side-effect the speed hack has is that the minimap has weird borders, but other than that, it shouldn't cause any other issues.