The ZooKeeper cache code caches Zookeeper stats. When a stat is requested from the cache it copies it. The ZK stat class offers no good way to copy other than serialize and deserialize. The code currently does this and its slow. All code in Accumulo only uses one field from stat, so it would be much better to create a simple class that has this one field and can quickly copy.
The stat is used very frequently in the metadata cache code to check if a tserver still holds its lock.