I don't know about the Glo HD but the card that came with my H2O, while only 4GB, it was not slow at all (read speeds 15-20MB/s). All my other cards I had at home were slower than the H2O card. So I don't think you will see a huge speed improvement in a normal use case.
I do notice significant slowdown when I use some of my (slow, old) cards so if you do replace it you should make sure it's at least as fast as the one that the Kobo came with.
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