I've been having a creative world I use for quite a while for different testing in redstone. I created this world when the 1.8 snapshots have started to release. I haven't got the strongest machine, but I've never experienced lag on this world. I'm not sure this is the cause, but as far as I can tell, ever since opening this world in 14w19a, I has been getting a lot of lag. This shows a bit in FPS, going down by about 5 (from 30 to 25), but mostly in redstone contraptions. A repeater on one tick takes about a second to light up, from the moment I connect it to a redstone power source. If I activate a more complex machine, in which a few things happen at once, the world just freezes, changing from one frame to another every few seconds, and I can only hear the sound of the pistons firing, letting me know something is happening. After I noticed that, I tried to open the world back in 14w18b, and this bug keeps happening in that version as well. However, I was not able to reproduce this bug in other worlds.
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It's just part of the snapshots. They are not very stable, and because of that, things like this happen a lot. This is also a reason why snapshots are made, people can report these things, so they can remove them for the release.
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There's some ongoing optimization work that should hopefully hit sometime in the next few weeks. This bug should probably stay open until the optimizations go into a snapshot, at which point lag can be re-evaluated to see if it's still there.
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Well, something unexpected just happened. I have discovered that my entire world was full with invisible WIther Skulls, and once I got rid of them the lag has gone. However, as far as I can recall, this appeared only after updating to the 14w19a snapshot, so it is either a coincidence or it is a very wired bug...
Confirming. Unusually strong lag occurs, mostly when I'm flying around in creative and rapidly drop altitude or something.