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Optimising The CRP

Optimising The CRP


July 25, 2016

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  2. Optimizing the CRP (Critical Rendering Path)

  3. Vlad Zelinschi Frontend Engineer at Yonder @vladzelinschi

  4. “One does not simply optimizes the critical rendering path so

  5. Reasons for being here • Easy to construct large apps

    and just … “go with the flow” • Performance should be in your head right from the start (this does not mean over-optimizing early on!!) • Demanding clients, demanding users, demanding developers, high standards

  7. What is the CRP?

  8. Google Developers “[…] what happens in these intermediate steps between

    receiving the HTML, CSS, and JavaScript bytes and the required processing to turn them into rendered pixels - that’s the critical rendering path.”
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  10. Candy break …

  11. Enter The Network … • DNS Lookup to resolve the

    hostname to IP address • New TCP connection (if necessary) • TLS/SSL handshake happening (worst case scenario) • HTTP request for index.html
  12. Some TCP knowledge • TCP is designed to probe the

    network to figure out the available capacity • TCP does not use the full capacity from the start • TCP Slow Start - for congestion control and avoidance • For the first roundtrip ~ 14KB (10 TCP packets)
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  14. let’s get the index.html

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  17. What browsers do … • Conversion: from raw bytes to

    individual characters (based on encoding - ex: UTF-8) • Tokenizing: converts strings of characters into distinct tokens specified by the W3C HTML5 standard • Lexing: tokens are converted into “objects” which define their properties and rules • DOM construction: relation between tags, tree-like construction
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  19. same for CSS …

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  24. then the render tree …

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  26. Render, Layout, Paint • Final stages before getting something on

    screen • The Layout calculates the exact position and size of the elements in the viewport • The render tree construction, position and size calculation are captured with the “Layout” event in the Timeline (Chrome Dev Tools) • Paint - converts the render tree to actual pixels
  27. Recap • Process HTML markup and build the DOM tree

    • Process CSS markup and build the CSSOM tree • Combine the DOM and CSSOM into a render tree • Run layout on the render tree to compute geometry of each node • Paint the individual nodes to the screen
  28. Optimizing the critical rendering path is the process of minimizing

    the total amount of time spent in steps one through five in the previous slide.
  29. Perf rules to keep in mind • HTML is parsed

    incrementally • CSS is render blocking (can’t construct the CSSOM and the render tree without it) • CSS is not incremental (we must wait for the entire file to download)
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  31. What about JavaScript?

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  33. JavaScript - friend and foe • The location of the

    script in the document is significant • DOM construction is paused when a script tag is encountered and until the script has finished executing (JS is parser-blocking) • JavaScript can query and modify the DOM and CSSOM • JavaScript execution is delayed until the CSSOM is ready
  34. How can we have pixels on screen after just one

    RTT (round trip time)?
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  39. Optimizing the CRP • Get CSS down to the client

    as fast as possible • Eliminate blocking JavaScript from the CRP (your friend, async) • Aim for the fastest domContentLoaded time (DOM and CSSOM are ready)
  40. Optimizing the CRP • Concentrate on the above the fold

    content - ATF (what the user sees first) • Inline CSS and JS for ATF (remember the 14KB limit?) • Put your critical assets on the same domain • Consider minifying HTML
  41. Tools for the trade • https://github.com/filamentgroup/grunt-criticalcss • https://github.com/bezoerb/grunt-critical • https://github.com/filamentgroup/loadCSS

    • https://github.com/filamentgroup/loadJS • https://developers.google.com/speed/pagespeed/
  42. Real world example https://www.fasetto.com/

  43. Thank you!

  44. Q&A anyone?

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  46. Optimizing the CRP (Critical Rendering Path) Vlad Zelinschi Frontend Engineer

    at Yonder Please fill the online evaluation form after event