When using fast graphics, leaves continue to use fancy graphics texture. The transparent spaces on the graphic allows the light of the world beyond through, meaning one can look through all the blocks as if it were sky, which can cause severe lag in systems such as mine, especially in forest areas, where it's most prominent and noticeable.

The graphics revert back to normal when the chat is shown, or whenever the inventory is hidden - and it doesn't matter what item/block you are holding. It remains glitched even when paused or when viewing the F3 debug screen. It occurs with all leaf block types, and can even be seen in the creative and survival inventory, as well as when held.

The issue has been present for a number of snapshots, including the latest one. The earliest I'm aware of is 14w28a, but I'm not sure how far back it started. It is not present in 1.7.