Player Created Structures Not Loaded Into Game.

When I loaded and joined my world, One structure was completely gone, another was partially missing. The attached picture shows two empty spaces. one contained a train car, which is now partially missing. The other, contained a red caboose, which was my home. I checked my world thoroughly and these were the only structures missing. Also, on a side note, rendering was somewhat slow when I loaded and joined my world. My questions are: 1. Why are my structures partially missing/gone? 2. Why is rendering so slow?