Implementing paging using range queries, latency of last query is much higher

var startTime = ISODate("2017-09-18T00:00:00Z");

var endTime = ISODate("2017-09-18T00:30:00Z");

var pageSize = 555;

var maxTimeMS = 6000;

var dbName = "foo";

var totalLatency = 0;

var skip = 0;

var lastTime = null;

var lastEventId = null;

var firstPage = true;

while (true) {

  var start = new Date();

  var eventsRead = 0;

  if (firstPage) {

    firstPage = false;

    db.getSiblingDB(dbName).events.find({ time: { $gte: startTime, $lte: endTime}}).sort({ "time" : -1 , "_id" : -1}).limit(pageSize).maxTimeMS(maxTimeMS).forEach(function(s) {

      eventsRead++;

      lastTime = s.time;

      lastEventId = s._id;

    } );

  } else {

    db.getSiblingDB(dbName).events.find({ time: { $gte: startTime, $lte: endTime}, "$or" : [ { "time" : { "$lt" : lastTime}} , { "time" : lastTime , "_id" : { "$lt" : lastEventId }} ] }).sort({ "time" : -1 , "_id" : -1}).limit(pageSize).maxTimeMS(maxTimeMS).forEach(function(s) {

      eventsRead++;

      lastTime = s.time;

      lastEventId = s._id;

    } );

  }

  var elapsed = new Date() - start;

  totalLatency += elapsed;

  skip = skip + eventsRead;

  print("page " + skip + ", elapsed " + elapsed);

  if (eventsRead < pageSize) {

    break;

  }

}

print("Total events read: " + skip + ", total latency: " + totalLatency);