There are a lot of books, articles, and online tutorials out there with fantastic advice on how to make your websites performant. It all seems easy in theory, but applying best practices to real-world code is anything but straightforward. Diagnosing and fixing frontend performance issues on a large legacy codebase is like being an archaeologist excavating the remains of a lost civilization. You don’t know what you will find until you start digging! Pick up your trowels and come along with Etsy’s Frontend Systems team as we become archaeologists digging into frontend performance on our large, legacy mobile codebase. I’ll share real-life lessons you can use to guide your own excavations into legacy code: What tools and metrics we used to diagnose issues and track progress. How we went beyond server-driven best practices to focus on the client. Which fixes successfully increased conversion, and which didn’t. Our work, like all good archaeology, went beyond artifacts and unearthed new insights into our culture. We at Etsy pride ourselves on our culture of performance, but, like all cultures, it needs to adapt and reinvent itself to account for changes to the landscape. Based on what we’ve learned, we are making the case for a new, organization-wide, frontend-focused performance culture that will solve the problems we face today. Presentation from https://perfmattersconf.com/

1. STARRING KATIE SYLOR-MILLERAND ETSY FRONTEND SYSTEMS

2. Katie Sylor-Miller
Staff Software Engineer
Etsy Frontend Systems
@ksylor && @ohshitgit

5. WE DO NOT FOLLOW MAPS TO BURIED TREASURE,
AND “X” NEVER, EVER MARKS THE SPOT

6. Archaeology is the
study of human history
and culture through
physical remains

7. Performance
archaeology uncovers
insights into your
development culture

8. Survey
Excavate
Hypothesis
Interpret
Archaeology 101

9. YOU CALL THIS ARCHAEOLOGY?

10. Hypothesis
Image by HeritageDaily on Wikipedia

11. The Mobile Listing Page

12. Mobile Visits
67% 52%
Mobile GMS

13. RUM Dom Content Loaded Times
1000 2000 3000 4000 50000
DOMContentLoaded ms
Numberofvisits

14. Dom Content Loaded Times
15%
37%
24%
13%
6%
1000 2000 3000 4000 50000
DOMContentLoaded ms
Numberofvisits

15. Conversion Vs. Dom Content Loaded
1000 2000 3000 4000 5000
DOMContentLoaded ms
Conversionrate

16. 1000 2000 3000 4000 5000
Conversion Vs. Population
15%
37%
24%
13%
6%
DOMContentLoaded ms
Conversionrate

17. Improving the
performance of the
mobile listing page will
increase conversion
Hypothesis

18. FORTUNE AND GLORY, KID

19. Survey
Hypothesis

20. Performance Best Practices
Great server response times
Minify and gzip static JS and CSS assets
Resize and compress images
Responsive images
Use proper headers to take advantage of browser caching
Use dns-prefetch resource hints
Use HTTP/2 when available (not all of our CDNs support)
In-house RUM monitoring and alerting
3rd party synthetic monitoring service

21. NOW WHAT?

22. Critical Rendering Path
How quickly a user:
• receives confirmation that the page has started loading?
• sees the most important information on the page?
• can interact with the page?

23. NOW WHAT?

24. Ground Penetrating Radar
Image by Tapatio via Wikipedia

25. WebPageTest.org
Ground Penetrating Radar

26. The Mobile Listing Page

27. WebPageTest Results (Before)
iPhone 6 iOS9 - 3GSlow
(this is much worse than our typical user experience)
METRIC BEFORE AFTER CHANGE
TTFB 1.8s
Start Render 8.5s
DOM Content Loaded 12.1s
Speed Index 13,193
Time to First Interactive >19s
CSS size 98KB/0.62MB
JS size 386KB/1.46MB
Images 37
Cost $$$$-

28. PRETEND THIS DIDN’T HAPPEN

29. Waterfall (before)

30. Waterfall (before)
Five CSS files
Block CSSOM construction
and Start Render

31. Waterfall (before)
Related listings images
(below the fold)

32. Waterfall (before)
Two different sizes of shop avatar image
(1 above, 1 below the fold)

33. Waterfall (before)
Three JS files
Global base (XL), page-specific (L), category nav(S)
Block DOM Content Loaded

34. Waterfall (before)
Similar listing images
(waaaayyyyy below the fold)

35. Five CSS bg-images
One icon font

36. Main listing image is 36th!!!
Followed by more carousel images

37. Reviews images (from async content)
Logging
Logging
Logging

38. $(‘document’).ready()

39. Areas For Improvement
• Lazyload images:
Defer loading of “below the fold” images
• Reduce CSS File Size:
Combine CSS files and remove unused rules
• Switch to SVGs:
Replace CSS bg-images and icon fonts with SVGs
• Reduce JS File Size:
Remove unused Javascript

40. Survey
Excavate

41. Test Pits

42. Lazyload
Images
Test Pit #1

43. 37 Images
814KB

45. lazy loading synthetic tests
DOMContentLoadedms
1750
3500
5250
7000
Network Speed
Wifi 4G 3G
Before
After Lazy Loading

46. Reduce CSS
File Size
Test Pit #2

47. 5 CSS files
98KB/0.62MB

48. “Load only what you need”
- Yoav Weiss, yesterday

49. EASIER SAID THAN DONE

50. Automation to the Rescue?
• Selenium script opens the page(s) in a browser
• Run uncss (https://github.com/uncss/uncss)
• Output a new file with only the CSS in use
… But, we didn’t capture *all* states and had to keep adding
more every time we found a bug.

51. CSS Reduction Synthetic Tests
DOMContentLoaded(ms)
1750
3500
5250
7000
Network Speed
Wifi 4G 3G
Before
After CSS Reduction
After Lazy Loading

52. Lazyloading + CSS
Reduction
Experiment Results
User-facing experiment #1

53. RUM Results - Lazyloading + CSS Reduction
METRIC BEFORE AFTER CHANGE
TTFB 1.16s 1.18s 1.2%
DOM Content Loaded 3.3s 3.1s 6% ➡
Page Load 6.0s 5.2s 13.2% ➡

54. RUM Results - Lazyloading + CSS Reduction

55. Conversion

56. X MARKS THE SPOT

57. Switch to
SVGs
Test Pit #3

58. 5 bg-images
1 icon font

59. SVG Switch Synthetic Tests
Logged Out
DOMContentLoadedms
1500
3000
4500
6000
Wifi 4G 3G
Before
After SVGs
Logged In
1500
3000
4500
6000
Wifi 4G 3G
Before
After SVGs

60. SVG Switch
Results
User-facing Experiment #2

62. RUM Results - SVG Switch
METRIC BEFORE AFTER CHANGE
TTFB 1.17 1.17s ⬌
DOM Content Loaded 3.0s 3.0s ⬌
Page Load 5.6s 5.5s 1%➡

63. RUM Results - SVG Switch

64. Conversion ⬌

65. WebPageTest.org
again
How are we doing?

66. METRIC BEFORE AFTER CHANGE
TTFB 1.8s 2.0s 11%
Start Render 8.5s 4.8s 44% ➡
DOM Content Loaded 12.1s 12.6s ?? 4%
Speed Index 13,193 13,071 ⬌
Time to First Interactive >19s >17s 10%➡
CSS size 98KB/0.62MB 38KB/0.24MB 60%➡
JS size 386KB/1.46MB 375KB/1.59MB 3%
Images 37 25 32%➡
Cost $$$$- $$$-- $➡
WebPageTest Results (After)
iPhone 6 iOS9 - 3GSlow

67. Waterfall - After
Down to 2 CSS Files!

68. Waterfall - After
Listing carousel images!

69. Waterfall - After
One shop avatar image!

70. Waterfall - After
Still two large JS files

71. Waterfall - After
CSS Background image
added back in by experiment

72. Logging
Reviews images (added by new experiment)
Async content

73. Stratigraphy

74. A Brief History of JS at Etsy
2010 - jQuery w/Homegrown system to
concatenate and serve JS files
2011 - Dependencies get inlined using rails-style
//=require ‘path/to/dependencies’
window.Etsy.module;
2012 - RequireJS - AMD dependencies
define([‘path/to/dependencies’],
function(dep) { });
2017 - still a mix of everything :(

75. HA HAAAAAHA

76. Reduce JS
File Size
Test Pit #4

77. “Load only what you need”
- Yoav Weiss, yesterday

78. 56KB/264KB
121 dependencies
142KB/712KB
124 dependencies
PAGE-SPECIFIC FILE GLOBAL BASE FILE

79. NO TYPES!

80. Manual JS
Reduction
Test Pit #4a

81. How to be a Javascript Detective
JS dependency
$(‘selector’)
Smarty/Mustache
templates
PHP View
PHP Controller
Experiment flags

82. 56KB/264KB
42KB/177KB
25% ➡
142KB/712KB
94KB/469KB
33% ➡
PAGE-SPECIFIC FILE GLOBAL BASE FILE

83. WebPageTest Results (After)
Manual JS removal
METRIC BEFORE AFTER CHANGE
TTFB 2.0s 1.9s 5%➡
Start Render 4.8s 4.87s 1%
DOM Content Loaded 12.6s 9.8s 22%➡
Speed Index 13,071 10,903 16%➡
Time to First Interactive >17s >15s 12%➡
CSS size 38KB/0.24MB 38KB/0.24MB ⬌
JS size 375KB/1.59MB 299KB/1.15MB 20%➡
Images 25 23 8%➡
Cost $$$-- $$$-- ⬌
iPhone 6 iOS9 - 3GSlow

84. Manual JS
Reduction Results
User-facing Experiment #3

86. RUM Results - Manual JS Reduction
METRIC BEFORE AFTER CHANGE
TTFB 1.2s 1.2s ⬌
DOM Content Loaded 2.9s 2.8s 2.9%➡
Page Load 5.6s 5.5s 1.6%➡

87. RUM Results - Manual JS Reduction

88. Conversion ➡

89. Javascript
Instrumentation
Test Pit #4b

90. Code Coverage?

91. window.Etsy =
window.Etsy || {};
window.Etsy.Vimes.register
(“vb383”);
function vb383(id,fn) {
return function() {
window.Etsy.Vimes.start('vb383'
, id);
return fn.apply(this,
arguments);
};
}
return vb383(1, function(args)
{
return stuff;
}
Re-write JS to log
function calls
Capture real user
actions
Send logs onLoad
and onUnLoad
Map # of calls back
to source files
Introducing Vimes

93. PAGE-SPECIFIC FILE
56KB/264KB
40KB/173KB
28% ➡
142KB/712KB
89KB/448KB
37% ➡
GLOBAL BASE FILE

94. Instrumented JS
Reduction Results
Fourth Experiment

95. RUM Results - Instrumented JS
METRIC BEFORE AFTER CHANGE
TTFB 1.2s 1.2s ⬌
DOM Content Loaded 3.0s 2.9s 2.7%➡
Page Load 5.2s 5.1s 1.2%➡

96. RUM Results - Instrumented JS

97. Conversion

98. Excavate
Interpret

99. Performance directly
affects conversion

100. Frontend performance
is just as important as
backend, if not more.

101. PUT IT IN THE BROWSER AND FORGET IT

102. Our experiences are not
our users experiences
(especially on mobile)

103. Small-scale tweaks
don’t always pan out

104. Your front-end architecture
needs to match your
development culture

105. Static top-level
CSS or JS
dependency
managed at the
page level
Dynamic FE
dependency
management at
the component
level
WHAT WE HAVE WHAT WE NEED

106. Architect for deletion

107. THANK YOU