To reproduce:

• Go to a tower in the End, in Creative Mode, and have the Shift+F3 graph open.
• Destroy the bedrock below the crystal (being careful not to destroy the crystal).
• You'll see the Fire block will become 1-sided (facing south), as it isn't resting on anything on its bottom (may or may not be relevant).
• You'll see the Fire block disappear and appear rapidly, causing the light to flicker rapidly.
• The 'Tick' portion of the pie will increase, and you will start to experience lag.
• Quit to the launcher, and in the Dev Console you will see a lot of
  • "Can't Keep Up!"
  • "Something's taking too long!"

You can make the lag go away by putting a block (doesn't have to be bedrock) back under the crystal.

Didn't occur before the 1.8 snapshots. The presence of the crystal prevented a Creative player from hitting it.