A running primary node' throughput degrades over time. Switching primary<->secondary solved the problem. As a precaution, we proactively shutdown the old primary once its secondary so that its ready for the next switch. MMS graph showing the degradation attached. This happens to primary nodes running WiredTiger. MMAP is not affected. We ran into this when we re-synced one of the secondaries nodes with storage engine set to WT and promoted it as primary