As MXHR responses grow larger, it becomes necessary to process each resource as it is received, rather than waiting for the entire response. This can be done by listening for readyState 3:

getLatesPacketInterval.js

var req = new XMLHttpRequest();
var getLatestPacketInterval, lastLength = 0;
req.open('GET', 'rollup_images.php', true);
req.onreadystatechange = readyStateHandler;
req.send(null);
function readyStateHandler{
 if (req.readyState === 3 && getLatestPacketInterval === null) {
 // Start polling.
 getLatestPacketInterval = window.setInterval(function() {
 getLatestPacket();
 }, 15);
 }
 if (req.readyState === 4) {
 // Stop polling.
 clearInterval(getLatestPacketInterval);
 // Get the last packet.
 getLatestPacket();
 }
}
function getLatestPacket() {
 var length = req.responseText.length;
 var packet = req.responseText.substring(lastLength, length);
 processPacket(packet);
 lastLength = length;
}