what does that mean? COMMAND [PeriodicTaskRunner] task: UnusedLockCleaner took: 273ms
The nice part about MongoDB is that it is open source, so your answer is in  the code, starting with comments about UnusedLockCleaner:


  Periodically purges unused lock buckets. The first time the lock is
  used again after cleanup it needs to be allocated, and similarly,
  every first use by a client for an intent mode may need to create a
  partitioned lock head. Cleanup is done roughtly once a minute.


Then taking a look at the lock_manager code we get more info about the cleanup function:


  Iterates through all buckets and deletes all locks, which have no requests on them. This
  call is kind of expensive and should only be used for reducing the memory footprint of
  the lock manager.


Basically it looks like some background maintenance being done by the database to clear out unused locks and keep the memory usage of the lock manager under control. Nothing to be concerned about, since if the database is busy and the locks are in use they won't be cleaned up. If you are seeing (reproducible) issues with it, then it would be best to open a ticket to debug with the developers, since this would not be intended.