Server crashing, large sums of entities.

My friend was on my server, farming sugar cane. He is experiencing a large amount of lag, so he checks his F3 and finds that over 2000 entities are being rendered. He chops the center of a growth of sugar cane, and the dropped cane "takes ages to pick up." 20 to 30 seconds after he joined the game, the server TPS drops to 1/60th, or 1 tick per minute. The following error message – 22.08 20:05:35 [Server] INFO [20:05:35] [Server Watchdog/FATAL]: A single server tick took 60.00 seconds (should be max 0.05) – appeared in the console. I'm not sure how to reproduce this; I doubt it's due to sugar cane farming but feel free to try it, . Here's a crash report! http://pastebin.com/EvZWFBHc