All queries waiting for lock in mongodb
In MMAPv1 write locks are collection level (version 3.0 and forward; Before 3.0, locks where database level). So, if you have 1174 clients reading "db.collection" and one client who is writing that same collection, readers will wait! Especially if you have write concern w:2 (or w:"majority") and you (only) secondary is lagging.

With wiredTiger locking mechanism is on document level.

You check here how concurrency works.