When several repeating command blocks are active in a world, it creates block update lag. It can take anywhere between 5-30 secs, sometimes more, for Minecraft to place/remove block textures.
Comments 3
I need to go back to what I originally said. The problem is in fact repeating command blocks, and the problem occurs when importing worlds from 1.2. I did manage to find a workaround for it. I destroyed the repeating commands blocks, put them back down, and refilled the command lines. That seemed to allow blocks to update again. So, something must be happening during import that updating the problematic blocks seems to "fix".
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Upon further investigation, it doesn't seem to be repeating command blocks, but just command blocks in general. And it doesn't matter what type of command block it is, either. If there's several of them in a small space, blocks don't update when they're supposed to.