Capped collection writer (includes insert, updates & delete ops) are serialized using a special resource metadata lock to maintain insertion(natural) order across the replica set. This lock is acquired after acquiring the write ticket and reserving the oplog slot  (fixed by SERVER-82180) However, heavy write workloads on capped collections can negatively impact other workloads in the system by depleting write tickets or causing repl lag (due to oplog hole). 

However, we've recently encountered two incidents on older mongod versions (5.0 and 7.0)with different customers where heavy capped write workloads led to ticket depletion and affecting other workloads in the system.

After PM-2983 which ensures natural ordering for all collections, including capped collections, the serialization of capped collection writers and the use of the resource mutex lock are no longer necessary. This feature is targeted 8.1 and will not be backported.