If I put a high continuous write load onto a a replica set I fall behind on the oplog until I fall off the end.

All three nodes are the same (High) spec - using write concern defaults and a multi threaded insert only workload ( OPLOG, Journal and Collection on same Drive). They are all interconnected by the same network, as is the client.

I know I could use a higher write concern but this is a simple and likely test scenario and I would expect the replication mechanism to be as fast as any external loader.