Investigate the hot-keys problem
Hot Keys
Sharded data structures are somewhat vulnerable to hot keys. I.e. a single key which is frequently operated on. In this case there will be high contention on the shard for which this key belongs to. This can be mitigated by introducing a non-determinstic sharding function which places hot keys in more than 1 shard. This solution does complicate things though, and introduces a probabilistic component to the data structure (e.g. look ups may result in the key not being found, when in fact it is actually in another shard. This trade off is generally acceptable in cache scenarios, where failed look ups just result in the key being re-populated).
Source: http://quinnftw.com/sharding-to-reduce-mutex-contention/
(This seems like something that might be happening in the tree structure like DNS itself for the portions of the hierarchy close to the "trunk".)