The document discusses optimizing user interaction performance in web applications by addressing issues such as slow interactions, browser rendering, and the impact of UI changes. It provides strategies for improving perceived performance, including offloading tasks from the main thread, optimizing animations, and careful use of layers. Additionally, it highlights potentially dangerous UI patterns and recommends testing tools for performance analysis.

1. Front-end Dev & Designer
at Lunar Logic
Anna Migas
Google Developer Expert
WebMuses core team

2. Anna Migas
@szynszyliszys
Fast but not furious:
Debugging user interaction
performance issues

3. Starability.css
Repaintless.css Auroral.css

4. Perceived performance
First Meaningful Paint
Time to interactive

5. Optimise Critical Rendering Path

6. Optimise Critical Rendering Path
Use progress bars instead of spinners

7. Optimise Critical Rendering Path
Use progress bars instead of spinners
Have a placeholder content

8. Optimise Critical Rendering Path
Use progress bars instead of spinners
Have a placeholder content
Start upload before user even
decides to click the upload button

9. Perceived Performance?

11. !
"

12. Perceived Performance

13. LOADPerceived Performance

14. !
"
Perceived load performance

15. !
Perceived performance
!

16. Perceived performance
First Meaningful Paint
Time to interactive
Smooth at all times

17. Agenda
1. Why sometimes interactions feel slow
2. Browser rendering and frames
3. Types of triggered changes in the UI
4. How to test for interaction performance
5. Potentially dangerous user interface patterns

18. J A N K

19. zdjęcie krzeseł
https://jakearchibald.github.io/jank-invaders/

20. 60 fps
1000/60 = ~16.6ms

21. 60 fps
1000/60 = ~16.6ms
120 fps
1000/120 = ~8.3ms
iPad Pro, Razer smartphone

22. How can we help the browser
to make it on time?

23. Offload the main thread
BUS
main thread (UI)
background thread (heavy task)

24. Offload the main thread
1. Use Web Workers for heavy tasks
2. Optimise the main thread

25. Offload the main thread
1. Use Web Workers for heavy tasks
2. Optimise the main thread

26. If we interact with a page, a browser
needs to start serving frames
(ideally 60 or 120 per second)

27. What a frame consists of?

28. What a browser does during these
steps?

29. Change happens, an event is fired
1

30. Styles are recalculated
2

31. Layout is calculated
3

32. Layout

33. Everything is rasterised
and painted to the layers
4

34. Paint

35. Paint

36. Layers

37. What creates new layers?
1. 3D or perspective transforms
2. Animated 2D transforms or opacity
3. Being on top/a child of a compositing layer
4. Accelerated CSS filters
5. In special cases <video>, <canvas>, plugins
6. will-change CSS property

38. .btn {
background-color: white;
border: 1px solid blue;
will-change: transform;
}
.btn:hover {
transform: scale(1.1);
}

39. Every layer consumes memory.
Use them wisely.

40. Compositing
5

41. Compositing

42. Compositing

43. Types of triggered changes

44. Layout change (Reflow)
(width, margin-top, left)

45. Paint change (Repaint)
(background-color, box-shadow, background-image)

46. Compositing change
(transform, opacity)

47. How to test if we have a problem?

48. Reflows

49. Reflows can be quite obvious…

50. …but for Repaints we might need
better tools

51. …but for Repaints we might need
better tools

52. Performance tab (Chrome or Firefox)

53. Layers tab (Chrome)

54. Rendering tab (Chrome)

55. Rendering tab (Chrome)

56. Paint flashing (Firefox)

57. Performance monitor tab (Chrome)

58. What are the potentially dangerous
UI patterns?

59. Animations
1

60. Animations
1. Don’t overuse animations
2. Animate only transform and opacity if possible
3. For 120fps avoid requestAnimationFrame()
4. Don’t animate elements below other nodes (like
fixed headers)
5. Don’t animate too many elements

61. Parallax effect
2

62. http://davegamache.com/parallax/

63. Parallax effect
1. Causes Paint Storms
2. Almost always bad
3. Don’t use scroll events
4. Don’t use background-position
5. Use 3D transforms and perspective if you need
to: https://developers.google.com/web/updates/
2016/12/performant-parallaxing

64. Fixed elements
3

65. https://jsipsum.lunarlogic.io/

66. Fixed elements
1. Repaints with every frame when scrolling
2. Use will-change property (or transform:
translate3D(0,0,0)) to avoid repainting
.fixed-element {
position: fixed;
will-change: transform;
}

67. Scrolling events
4

68. Scrolling events
1. Don’t attach wheel or touch listeners to the whole
document, use smaller areas instead
2. Take advantage of passive event listeners (use
with polyfill):
window.addEventListener("scroll", func, {passive: true});

69. Hover effects
5

71. Hover effects
1. If they are bound to happen often—you might
consider using will-change property
2. Can be deadly if there are too many and can be
easily triggered
3. Avoid effects triggering Layout or Paint:
https://csstriggers.com/

72. Appending elements
6

74. Appending elements
1. Make sure not many elements will be affected
2. Try to separate the area (will-change, contain:
layout)
3. Try not to change the size of area (for example
use overflow property)

75. Summary
1. Jank happens when the browser doesn’t finish
rendering a frame on time
2. Try to offload and optimise the main thread
3. Avoid content Reflows and Repaints
4. Don’t overuse layers
5. Test your website with Performance, Layers and
Rendering tabs
6. Use responsibly potentially dangerous UI patterns

76. Resources
1. http://jankfree.org
2. https://dev.opera.com/articles/css-will-change-property
3. https://www.udacity.com/course/browser-rendering-optimization--
ud860
4. https://www.html5rocks.com/en/tutorials/speed/layers
5. https://www.html5rocks.com/en/tutorials/speed/high-performance-
animations
6. https://blog.logrocket.com/eliminate-content-repaints-with-the-new-
layers-panel-in-chrome-e2c306d4d752

77. Thanks!
Anna Migas
@szynszyliszys