Severe frame drop and crashing after auto-save

Pre-release 1 world is stuck in a 'Low framerate state' from crash after autosave initiated. The world had been running fine at high framerate for the first 30 minutes, until the 5th or 6th autosave, and now that particular save game only manages 1-2 frames per second even after reloading, sometimes crashing altogether. Other worlds still work fine.