This document discusses debugging user interaction performance issues in web pages. It begins by explaining how the browser renders frames and what happens during each step of the rendering process. It then covers the different types of changes that can trigger layout, paint, or compositing updates. Potentially problematic UI patterns like animations, parallax effects, scrolling events and appending elements are described. The document provides tips on using tools like the Performance tab to test for interaction issues and recommends techniques like using will-change to avoid unnecessary repaints and reflows.

1. Front-end Dev & Designer
at Lunar Logic
Anna Migas
Google Developer Expert
@szynszyliszys

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

3. Starability.css
Repaintless.css Auroral.css

4. Perceived performance

5. Perceived performance
First Meaningful Paint
Time to interactive

6. 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

7. Perceived Performance

8. LOADPerceived Performance

9. !
"
Perceived load performance

10. !
Perceived performance
!

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

12. 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

13. J A N K

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

15. 60 fps
1000/60 = ~16.6ms

16. 60 fps
1000/60 = ~16.6ms
120 fps
1000/120 = ~8.3ms
iPad Pro, Razer smartphone

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

18. What a frame consists of?

19. What a browser does during these
steps?

20. Change happens, an event is fired
1

21. Styles are recalculated
2

22. Layout is calculated
3

23. Layout

24. Everything is rasterised
and painted to the layers
4

25. Paint

26. Paint

27. Layers

28. 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

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

30. Every layer consumes memory.
Use them wisely.

31. Compositing
5

32. Compositing

33. Compositing

34. Types of triggered changes

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

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

37. Compositing change
(transform, opacity)

38. How to test if we have a problem?

39. Reflows can be quite obvious…

40. …but for Repaints we might need
better tools

41. Performance tab (Chrome or Firefox)

42. Performance tab (Chrome or Firefox)

43. Layers tab (Chrome)

44. Rendering tab (Chrome)

45. Rendering tab (Chrome)

46. Paint flashing (Firefox)

47. Performance monitor tab (Chrome)

48. What are the potentially dangerous
UI patterns?

49. Animations
1

50. 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

51. Parallax effect
2

52. http://davegamache.com/parallax/

53. 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

54. Fixed elements
3

55. https://jsipsum.lunarlogic.io/

56. 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;
}

57. Scrolling events
4

58. 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});

59. Hover effects
5

61. 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/

62. Appending elements
6

65. 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)

66. Takeaways
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

67. 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
7. https://dassur.ma/things/120fps/

68. Thanks!
Anna Migas
@szynszyliszys