How to reproduce this bug (screenshots follow along):

1) Make a contraption similar to the first screenshot. Repeaters are on 4 ticks (max). The commands in the command blocks can just be /say hi or /say trains or whatever.

2) Put a redstone block as shown in the second screenshot, to activate the redstone repeaters. The contraption works as normal. Just like what anyone would expect.

3) Type a /setblock command to put a redstone block in the same place. Execute it, see it work again (if the previous block was destroyed), then destroy the newly placed block. So therefore, that command is saved in the chat history. (Screenshot 3)

4) Teleport away from the contraption so it isn't in sight (in my case, screenshot 4, I teleported 2000 blocks away).

5) Execute the same /setblock command. Now the text appears double the speed (faster than when the contraption was visible). That's the bug.

In my case the new VBO option doesn't change the output. Well, that's all. See if it's a true bug or my imagination.