var startTime = Date.now();
var count = 0;
var files = DriveApp.getFiles();
while (files.hasNext()) { count++; var file=files.next();};
var endTime   = Date.now();
Logger.log('Loop through ' + count + ' files takes ' + ((endTime-startTime)/1000) + ' seconds');

var startTime = Date.now();
var fileIds = getFileIds(); // retrieve info cache (stored before)
var count = 0;
var numFiles = fileIds.length;
  for (var i=0; i<numFiles; i++) { count++; var file = fileIds[i];};
var endTime   = Date.now();
Logger.log('Loop through ' + count + ' files takes ' + ((endTime-startTime)/1000) + ' seconds');

var files = DriveApp.searchFiles(criteria);