1. @tameverts
#PerfNow
How I learned
to stop worrying
and love UX metrics

2. @tameverts
WPOstats.com

3. What is the best UX metric?
How fast should it be?
How can we stay on track?

5. https://www.cnet.com/news/appliance-science-the-well-done-physics-chemistry-of-the-toaster/

9. TTFB DNS TCP Start render
DOM loading DOM ready Page load Fully loaded
TTI Resource timing Number of requests Bytes in
Speed Index PageSpeed Navigation timing DOM elements
DOM size Visually complete TTFP TTFMP

10. “The real thing we are after is
to create a user experience that
people love and they feel is fast…
and so we might be front-end
engineers, we might be dev,
we might be ops,
but what we really are is perception
brokers.”
Steve Souders

11. How do we measure
perception… at scale?

12. Is it happening?
Is it useful?
Is it usable?
Is it delightful?
https://developers.google.com/web/fundamentals/
performance/user-centric-performance-metrics

14. TTFB DNS TCP
TTI FCP FMP
FID OMG WTF

15. ❑ Correlates to what users actually see in the browser
❑ Is easy to use and accessible right out of the box
❑ Recognizes that not all pixels and page elements are
equal
❑ Allows us to customize what we measure on specific
pages
The best UX metric…
#PerfNow@tameverts

17. A brief history
of performance metrics

18. Load Time
the time from the start of the initial
navigation until the beginning of the
window load event

19. BBC loses 10% of users for every additional
second it takes their site to load
Ancestry.com saw a 7% conversion rate
increase after improving load time by 64%
FT.com reduced desktop load times to
1.5s and mobile to 2.1s and increased
user engagement by 30%
@tameverts #PerfNow

20. Start Render
the time from the start of the initial
navigation until the first non-white
content is painted

21. start render repeat visits

22. wow!

23. @tameverts #PerfNow

24. Speed Index
average time at which visible parts of
the page are in the viewport

25. When good metrics go bad…

30. First Paint
First Contentful Paint
First Meaningful Paint
First CPU Idle
First Interactive

31. First Paint (FP)
Pixels first start to render

32. First Contentful Paint (FCP)
Text and graphics start to render…
BUT often catches non-meaningful paints
(e.g. headers, nav bars)
https://tinyurl.com/paint-metrics

33. First Meaningful Paint (FMP)
The paint after which the biggest
ATF layout change has happened
and web fonts have loaded
https://tinyurl.com/paint-metrics

35. https://speedcurve.com/blog/an-analysis-of-chromiums-paint-timing-metrics/

36. “The rendering pipeline is very complex, and
the timestamp should be the latest timestamp
the browser is able to note in the pipeline (best
effort).
Typically the time at which the frame is
submitted to the OS for display is
recommended for this API.”
https://w3c.github.io/paint-timing/#sec-terminology

37. Analysis of 40 top Alexa-ranked sites
95% of FP events occur before Start Render
85% of FCP events occur before Start Render
50% of FMP events occur before Start Render
https://speedcurve.com/blog/
an-analysis-of-chromiums-paint-timing-metrics/

38. ❑ Correlates to what users actually see in the browser
❑ Is easy to use and accessible right out of the box
❑ Recognizes that not all pixels and page elements are
equal
❑ Allows us to customize what we measure on specific
pages
The best UX metric…

39. First CPU Idle
(formerly known as First Interactive)
Page is minimally interactive
Most visible UI elements are interactive
Responds to user input reasonably quickly

40. Time to Interactive (TTI)
(formerly known as Time to Consistently Interactive)
Displays useful content
Event handlers are registered for most visible elements
Page responds to user interaction within 50ms

42. ❑ Correlates to what users actually see in the browser
❑ Is easy to use and accessible right out of the box
❑ Recognizes that not all pixels and page elements are
equal
❑ Allows us to customize what we measure on specific
pages
The best UX metric…

43. Okay… so then what?

44. Hero Rendering Times
H1
Largest Image
Largest Background Image
First Painted Hero
Last Painted Hero
https://speedcurve.com/blog/web-performance-monitoring-hero-times/

48. One-size-fits-most (not all)
Dependent on how page is built
Don’t work for image carousels and
popups
Hero times gotchas

49. So which metric is best?

50. http://lab.speedcurve.com/rendering/picker.php

51. And the winner is…

52. It depends!
(You probably saw that coming.)

53. Deliver any content?
Start Render
Deliver significant
amount of content?
Speed Index, FMP
Deliver critical content?
Hero Rendering Timeshttps://speedcurve.com/blog/rendering-metrics/

54. ❑ Correlates to what users actually see in the browser
❑ Is easy to use and accessible right out of the box
❑ Recognizes that not all pixels and page elements are
equal
❑ Allows us to customize what we measure on specific
pages
The best UX metric…

55. Custom metrics
Measure performance with high-precision timestamps
Available in both synthetic and RUM (yay!)
https://www.w3.org/TR/user-timing/
https://speedcurve.com/blog/user-timing-and-custom-metrics/

56. how long does it take
to display the main
product image on my
site?

57. Time to First Tweet
The time from clicking the link to viewing
the first tweet on each page’s timeline
Pinner Wait Time (PWT)
The time from initiating an action (e.g.,
tapping a pin) until the action is complete
(pin close-up view is loaded)
Time to Interact (TTI)
@tameverts #PerfNow

58. @tameverts #PerfNow

59. ❑ Correlates to what users actually see in the browser
❑ Is easy to use and accessible right out of the box
❑ Recognizes that not all pixels and page elements are
equal
❑ Allows us to customize what we measure on specific
pages
The best UX metric…

61. What else can we track?

62. First Click
First Scroll
First Key
First Input Delay
(only available with real user monitoring)

65. Great!
We have metrics.
Now… what to do with them?

66. Performance budgets FTW!

67. Thresholds YOU create for metrics
that are meaningful for YOUR site
https://addyosmani.com/blog/performance-budgets/
Milestone timings (e.g. start render)
Quantity-based (e.g. image weight)
Rules-based (e.g. Lighthouse scores)

68. A good performance budget
should show you…
What your budget is
When you go out of bounds
How long you’re out of bounds
When you’re back within budget

70. Recommended UX perf budgets*
Metric How to track it Budget
Start render Synthetic & RUM 2s
First Painted Hero Synthetic 2s
Speed Index Synthetic 4s
First Interactive Synthetic 4s
First Input Delay RUM 10ms
@tameverts *See super important stuff on the next slide

71. Super important!
Look at your own data
Monitor your competitors
No sandbagging allowed
Take a step-by-step approach if necessary
Use synthetic and RUM (numbers may will vary)

72. Pro tips
Create budgets for your popular and regularly
changing pages
Review violations early and always
Compare before and after releases
Update budgets accordingly
https://www.zillow.com/engineering/bigger-faster-more-engaging-budget/

73. Takeaways

74. 1. There is no unicorn metric
2. Use your eyes
3. Don’t just look at averages and medians
4. Validate your metrics by correlating them with business
and engagement KPIs
5. Set targets (e.g., performance budgets)
6. Make yourself accountable (e.g., alerts)
7. Don’t be afraid to investigate new metrics!
@tameverts #PerfNow

75. thanks!
@tameverts
speedcurve.com/blog