I don't think using a class 6 card is that much faster (or even at all) than using a class 4 card. I can't see any difference in speed in the UI - only in load times.
Current phone: Google Nexus 5 (32GB Black)| Previous phone:
HTC One - Silver/White (64GB), HTC One - Silver/White (32GB) - sold.
Current tablet: Google Nexus 7 (32GB - CA0)