Since a few days ago (January 14th) we're seeing spikes in write lock % tracked in MMS, but only on one shard out of 5 (our _c shard). We're also seeing spikes in read queue at the same time write lock % spikes.
There are a few non-sharded collections having this shard as their primary, but they don't have that many writes.

However, we don't see spikes in IO / CPU load or network traffic on those servers, so we don't understand the exact cause of those spikes.
Can anybody explain what could be the bottleneck other than disk IO, CPU, or network bandwidth?