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Performance Tools

A variety of tools, mainly tools already existent in the browser

Some background

First, some time stamps that the browser keeps track of when loading your page:

navigation timing api timestamps

It goes like this:

  1. domLoading - start parsing the first received bytes of the HTML document
  2. domInteractive - point when the browser has finished parsing all of the HTML and DOM construction is complete
  3. DOM is ready
  4. domContentLoaded - point when DOM is ready and there are no stylesheets blocking JS execution
  5. DOM and CSSDOM are ready
  6. domComplete - all of the processing is complete and all of the resources on the page (images, etc.) have finished downloading
  7. loadEvent - a final step in every page load the browser fires an onload event which can trigger additional application logic.
      function measureCRP() {
        const t = window.performance.timing;
        const interactive = t.domInteractive - t.domLoading;
        const dcl = t.domContentLoadedEventStart - t.domLoading;
        const complete = t.domComplete - t.domLoading;
        let stats = document.createElement('p');
        stats.textContent = 'interactive: ' + interactive + 'ms, ' +
            'dcl: ' + dcl + 'ms, complete: ' + complete + 'ms';
  <body onload="measureCRP()">
    <p>Hello <span>web performance</span> students!</p>
    <div><img src="awesome-photo.jpg"></div>

Above captures when the API’s events are triggered.

performance.getEntriesByType('resource' || 'navigation')

The following section is focused on the data returned from this function call

// Get Navigation Timing entries:
performance.getEntriesByType('navigation'); // HTML documents, etc.

// Get Resource Timing entries:
performance.getEntriesByType('resource'); // document dependent resources

Both return arrays containing an object similar to this:

  "connectEnd": 152.20000001136214,
  "connectStart": 85.00000007916242,
  "decodedBodySize": 1270,
  "domComplete": 377.90000007953495,
  "domContentLoadedEventEnd": 236.4000000525266,
  "domContentLoadedEventStart": 236.4000000525266,
  "domInteractive": 236.2999999895692,
  "domainLookupEnd": 85.00000007916242,
  "domainLookupStart": 64.4000000320375,
  "duration": 377.90000007953495,
  "encodedBodySize": 606,
  "entryType": "navigation",
  "fetchStart": 61.600000015459955,
  "initiatorType": "navigation",
  "loadEventEnd": 377.90000007953495,
  "loadEventStart": 377.90000007953495,
  "name": "",
  "nextHopProtocol": "h2",
  "redirectCount": 0,
  "redirectEnd": 0,
  "redirectStart": 0,
  "requestStart": 152.50000008381903,
  "responseEnd": 197.80000008177012,
  "responseStart": 170.00000004190952,
  "secureConnectionStart": 105.80000001937151,
  "startTime": 0,
  "transferSize": 789,
  "type": "navigate",
  "unloadEventEnd": 0,
  "unloadEventStart": 0,
  "workerStart": 0
  • This is what Google’s performance tab displays, a data visualisation of this data among other data.

DNS lookup time

When a user requests a URL, the Domain Name System (DNS) is queried to translate a domain to an IP address

  • domainLookupStart marks when a DNS lookup starts.
  • domainLookupEnd marks when a DNS lookup ends.
// Measuring DNS lookup time
var pageNav = performance.getEntriesByType('navigation')[0];
var dnsTime = pageNav.domainLookupEnd - pageNav.domainLookupStart;
  • Around 20 ms in the data above.

Connection negotiation

When a connection to a server is made, latency occurs as the client and server sort things out prior to sending resources to the client.

  • connectStart marks when the client opens a connection to the server.
  • secureConnectionStart marks when the client begins TLS (a cryptographic protocol that provides end-to-end communications security over networks) negotiation.
  • connectEnd marks when connection negotiation ends (including TLS time).
// Quantifying total connection time
var pageNav = performance.getEntriesByType('navigation')[0];
var connectionTime = pageNav.connectEnd - pageNav.connectStart;
var tlsTime = 0; // <-- Assume 0 by default

// Did any TLS stuff happen?
if (pageNav.secureConnectionStart > 0) {
  // Awesome! Calculate it!
  tlsTime = pageNav.connectEnd - pageNav.secureConnectionStart;
  • Around 67ms in the data.

Requests and responses

Some of the most revealing data is here.

There are two factors here:

  • Extrinsic factors: Stuff like connection latency and bandwidth. Things (mostly) out of our control as developers.
  • Intrinsic factors: What we have control over… server and client side architecture, and resouce size.

This data expresses how long each resource took to arrive.

  • fetchStart - when the browser starts to fetch a resource. This marks when the browser begins to check caches to see if a network request is necessary
  • workerStart - when a request is being fetched from a service worker within a fetch event handler. This is 0 if a service worker isn’t installed for the current page
  • requestStart when the browser issues the network request
  • responseStart when the first byte of the response arrives
  • responseEnd when the last byte of the response arrives


// Cache seek plus response time
var pageNav = performance.getEntriesByType('navigation')[0];
var fetchTime = pageNav.responseEnd - pageNav.fetchStart;

// Service worker time plus response time
var workerTime = 0;

if (pageNav.workerStart > 0) {
  workerTime = pageNav.responseEnd - pageNav.workerStart;

// Request plus response time (network only)
var totalTime = pageNav.responseEnd - pageNav.requestStart;

// Response time only (download)
var downloadTime = pageNav.responseEnd - pageNav.responseStart;

// Time to First Byte (TTFB)
var ttfb = pageNav.responseStart - pageNav.requestStart;

Document processing

Basically, how long it takes the browser to process the document.

  • domInteractive, domContentLoadedEventStart, domContentLoadedEventEnd, and domComplete.
  • Only Navigation Timing API


When a document and its resources have completely finished loading.

  • loadEventStart and loadEventEnd or duration

Document and resource size

The size of the document or resource.

  • transferSize is the size of the resource, including HTTP headers.
  • encodedBodySize is the compressed size, excluding HTTP headers.
  • decodedBodySize is the decompressed size, excluding HTTP headers.
// HTTP header size
var pageNav = performance.getEntriesByType('navigation')[0];
var headerSize = pageNav.transferSize - pageNav.encodedBodySize;

// Compression ratio
var compressionRatio = pageNav.decodedBodySize / pageNav.encodedBodySize;
  • Network panel tells you this stuff already though.