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.