MongoDB: replication headroom gone below 1 days (avg/sec)
"Replication headroom" means "how long time of changes your opLog can store". To get larger "replication headroom" you need to expand your opLog size at least one node, of course it is preferred to have same size opLog at every node.

Reason why this headroom has gone under one day, is that because you have more changes (opLog documents) nowadays, than before.