Glow Lichen can be harvested at a rate of 6400 items per hour, as shown in https://www.youtube.com/watch?v=TGPyaN0-J1E and as can be easily replicated by testing a bunch.

6400 items per hour equals 11.25 game ticks per item, which is clearly broken as it indicates a desync from the main tick loop, moreover there is absolutely no place where that number would come from:

It has a 6gt breaking time, one tick would be needed to regrow = 7gt/item
Taking non-instamining delay into account: 6gt breaking, 6gt delay, one tick regrow = 13gt/item

The Minecraft Vanilla Server generates 7200 plants per hour, not 6400; and latency is no excuse in singleplayer.

Edit: wrote up a gist explaining this: https://gist.github.com/ExaInsanity/e7004a3d11a811b3dabd02d7092151b7