When performing a mongodump on an empty database, with a large oplog file, the oplog dump process takes considerably longer than one might expect. In our case, it took 3 minutes.

2018-07-05T11:14:21.682-0500 writing admin.system.version to
2018-07-05T11:14:21.686-0500 done dumping admin.system.version (1 document)
2018-07-05T11:14:21.695-0500 writing captured oplog to
2018-07-05T11:17:57.781-0500 dumped 1 oplog entry