Launch mongod's: ./mongod --replSet foo --storageEngine wiredTiger --dbpath /mnt/work/data/rs0 --oplogSize 1000 --wiredTigerCacheSizeGB 4 --logpath /mnt/work/data/rs0/mongod.log --fork ./mongod --replSet foo --storageEngine wiredTiger --dbpath /mnt/work/data/rs1 --oplogSize 1000 --wiredTigerCacheSizeGB 4 --port 27018 --logpath /mnt/work/data/rs1/mongod.log --fork Setup the replset: rs.initiate() rs.add( "localhost:27018" ) Run javascript against the primary to seed documents var INT_MAX=10000000; for (x=0;x<INT_MAX/1000;x++){ var bulk = db.test.initializeUnorderedBulkOp() for (i=0;i<1000;i++){ var num = (x*1000)+i; bulk.insert({_id:num,x:num,y:num}); } bulk.execute(); } Execute the attached repro.rb Run the following javascript on the secondary to monitor performance. You should see an initially stable number of query times, which will then blow out to seconds and minutes: while ( true ){ var INT_MAX=10000000; start = new Date() rs.slaveOk() for (i=0;i<10000;i++){ var docNum = Math .floor( Math .random()*INT_MAX); db.test.findOne({_id:docNum}); } print( "Took: " +( new Date() - start) + "ms" ) sleep(1000); }