I feel like this is a major bug that needs to be addressed. Whenever I place or break a block after building a medium sized or larger structure, I start to get very bad block lag. This usually only happens to me offline. I can run MC perfect offline on normal with no lag. I don't understand why this happens but it has happened to me since I started playing (Minecraft 1.3) Many other players report this issue. Some have good computers, some have bad. I believe it is an issue with Minecraft itself. There are many different theories speculating this bug. It happens to myself in both Creative and Survival and greatly discourages me from playing. I will be getting 40+ fps, and, once a block is place, the game freezes for a split second and breaks 2-4 instead. The same happens when breaking blocks. It does not effect any other game aspects. Please, address this issue as soon as possible. Thanks