Minecraft stores all loaded chunks in a hash table. The hash function used starts by XOR'ing the chunk's X and Z coordinates, so that many nearby chunks will get the same hash. This causes chunk lookups - and block updates, block lookups, tile entity lookups, etc - to take O(sqrt(N)) time instead of O(1), where N is the number of loaded chunks.

For a server with 10 players and a view-distance of 10, there will be 4410 loaded chunks, and chunk lookups will be up to 66 times slower than they should be, or 20 times slower in the best case.