2020 Chrome Dev Summit: Web Performance 101

Web Performance 101:
What is web performance
and why should I care?
@tameverts
#ChromeDevSummit
¯_(ツ)_/¯

What is “web performance”?
Why should I care about it?
How do I measure it?
How can I get other people in my company
to care about it?

the average web user believes they waste
two days a year waiting for pages to load

“web stress”
When apps or sites are slow,
we have to concentrate
up to 50% harder to stay on task.
@tameverts

11
When do users start to interact with a page?

15
“We want you to be able to flick from one
page to another as quickly as you can flick a
page on a book.
So, we’re really aiming very, very high
here… at something like
100 milliseconds.”
Urs Hölzle
SVP Engineering, Google

Slow pages affect people’s perception
of three things completely unrelated to time:
1. Content “boring”
2. Visual design “tacky”
“confusing”
3. Ease of navigation “frustrating”
“hard-to-navigate”

User experience and
web performance
are predictable indicators
of business outcomes.

Rebuilding Pinterest pages for performance resulted in
40% decrease in wait time, 15% increase in SEO traffic,
and 15% increase in signup conversion rate.
Ancestry.com saw a 7% increase in conversions after
improving render time by 68%,
page weight by 46% and load time by 64%.
Staples reduced median page load time by
1 second and 98th percentile load time by
6 seconds, resulting in a 10% conversion rate increase.
@tameverts

optimal load times for peak conversions
@tameverts

even 100ms delays matter
@tameverts

real user data +
machine learning

collected 1M+ beacons of real user data
across 93 attributes, including…
• top-level – domain, timestamp, SSL
• session – start time, length (in pages), total load time
• user agent – browser, OS, mobile ISP
• geo – country, city, organization, ISP, network speed
• bandwidth
• timers – base, custom, user-defined
• custom metrics
• HTTP headers

sessions that convert
have fewer images

pages with more scripts
are less likely
to convert

160+ scripts… uh-oh
@tameverts

speakerdeck.com/csswizardry/its-my-third-party-and-ill-cry-if-i-want-to

“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

But measuring perception is hard.
It’s even harder to scale.

What tools do we use?
Synthetic (lab)
Consistent baseline
Mimics network & browser conditions
No installation
Compare any sites
Detailed analysis
Waterfall charts
Filmstrips and videos
Limited URLs
Real user monitoring (field)
Requires JavaScript installation
Large sample size (up to 100%)
Real network & browser conditions
Geographic spread
Correlation with other metrics (bounce rate)
No detailed analysis
Only measure your own site

Free tools to explore
Synthetic
webpagetest.org
developers.google.com/speed/
pagespeed/insights/
Real user monitoring
github.com/bluesmoon/boomerang
developers.google.com/web/tools/
chrome-user-experience-report

developers.google.com/speed/pagespeed/insights/

A brief history
of performance metrics

TTFB DNS TCP
TTI FCP FMP
FID OMG WTF

❑ 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…

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

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

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

First Paint
First Contentful Paint
First Meaningful Paint

First Paint (FP)
Pixels first start to render

First Contentful Paint (FCP)
Text and graphics start to render…
BUT often catches non-meaningful paints
(e.g. headers, nav bars)

First Meaningful Paint (FMP)
The paint after which the biggest
ATF layout change has happened
and web fonts have loaded

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

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

Speed Index
Average time at which visible parts of
the page are in the viewport

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/

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

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

Lighthouse
Scores based on audits run on synthetic tests.
Checks your page against “rules” for Performance, PWA, Best
Practices, and SEO.
For each category, you get a score out of 100 and
recommendations for what to fix.
developers.google.com/web/tools/lighthouse

matuzo.at/blog/building-the-most-inaccessible-site-possible-
with-a-perfect-lighthouse-score/

developers.google.com/search/blog/2020/11/timing-for-page-experience

“Core Web Vitals are the subset of Web Vitals that
apply to all web pages, should be measured by all site owners,
and will be surfaced across all Google tools.
“Each of the Core Web Vitals represents a distinct facet of
the user experience, is measurable in the field, and
reflects the real-world experience of a critical user-centric outcome.
“The metrics that make up Core Web Vitals will evolve over time.
“The current set for 2020 focuses on three aspects of the user experience —
loading, interactivity, and visual stability — and includes the following metrics…
web.dev/vitals/

Largest Contentful Paint
First Input Delay
Cumulative Layout Shift
loading
interactivity
visual stability

75th percentile of page loads
across mobile and desktop

Amount of time it takes for the largest visual element to render.
Available in Chrome and Chromium-based browsers.
Measurable via synthetic and RUM.

Amount of time it takes for page to respond to user input (e.g.
click, tap, key).
Only measurable via RUM.

FID can seem fast because user interactions
take place later in the page’s rendering cycle...
after CPU-hogging long tasks have completed.
speedcurve.com/blog/first-input-delay-google-core-web-vitals/

No correlation when looking at all sessions

Stronger correlation at 75th percentile

Long Tasks have a high correlation
across the board

Long Tasks
Measures JavaScript functions that take 50ms or longer.
Long or excessive JS tasks can delay rendering,
as well as cause page “jank”.

Score that reflects how much page elements shift during rendering.
Available in Chrome and Chromium-based browsers.

Size of the shifting element matters
speedcurve.com/blog/visualising-cls-layout-shifts/

Image carousels can generate false positives

Web fonts & opacity changes can cause issues

Bounce rate gets worse as CLS degrades
Bounce rate improves as CLS degrades
Bounce rate stays the same as CLS degrades
@tameverts

How do these metrics correlate with
my business goals?
How fast should they be?
How do we stay on track?

cnet.com/news/appliance-science-the-well-done-physics-chemistry-of-the-toaster/

How to create
a culture of performance

“The largest hurdle to creating and
maintaining stellar site performance
is the culture
of your organization.
Lara Hogan
designingforperformance.com

“No matter the size or type of team,
it can be a challenge to educate,
incentivize, and empower those around you.
“Performance more often comes down to
a cultural challenge, rather than simply
a technical one.”
Lara Hogan
designingforperformance.com

2009 Improved average load time from 6s  1.2s
7-12% increase in conversion rate + 25% increase in PVs
Average load time degraded to 5s
User feedback: “I will not come back to this site again.”
Re-focused on performance
0.4% increase in conversion rate
2010
2011
@tameverts

1. No front-end measurement
2. Constant feature development
3. Badly implemented third-parties
4. Waiting too long to tackle performance
problems
5. Relying on performance sprints

Making it up as you go
is not always a good idea.
(Actual photo taken yesterday
of my family’s gingerbread village.)

3. Then build a
cross-disciplinary team

Everyone who touches
a page should care
about the performance
of that page.

Embrace performance from the ground up.
Embed engineers into other teams.
Enlist performance ambassadors.
Teach people how to use (or at least understand) the
monitoring tools you use.

It’s perilously easy
to accidentally become a
gatekeeper.

We first went to the engineering
leaders,
and then we went to
our product leader.
Our pitch was
totally different...
Reefath Rajali // PayPal
chasingwaterfalls.io/episodes/episode-two-with-reefath-rajali/

“When we went to our product leaders,
we spoke more about the business numbers
and the business benefits.
“When we spoke to our engineering leaders,
it was more about our consumer delight.”
Reefath Rajali // PayPal
chasingwaterfalls.io/episodes/episode-two-with-reefath-rajali/

Find out what people
care about

❑ bounce rate
❑ cart size
❑ conversions
❑ revenue
❑ time on site
❑ page views
❑ SEO
❑ user happiness
❑ user retention
❑ competitors

If they care about business
metrics…

If they care about
user engagement…

If they care about
third parties…

Who they are What they care about What to show them
Executives
Competition
Business impact
Benchmarks (filmstrips and videos)
Correlation charts (perf + KPIs)
Marketing
Third parties
Traffic + engagement
SEO
Content
Third-party performance
Correlation charts (perf + bounce rate)
Lighthouse SEO audits
Image size
Devs / engineers Well, lots of stuff, probably Consult with perf team

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

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

zillow.com/tech/bigger-faster-more-engaging-budget/

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)

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

Who What Metric
Ops Back-end issues TTFB
Marketing
Most important content
Third parties
SEO
Largest Contentful Paint
JS Long Tasks
Lighthouse SEO score & audits
Devs / engineers
How well pages are built
Performance issues
Start Render, Web Vitals
Lighthouse Performance audits

“One of the original directives of the
performance team
was we weren’t going
to set ourselves up
to be performance cops.”
Dan Chilton, Vox Media
responsivewebdesign.com/podcast/vox-media-performance/

“We weren’t going to go around slapping people on the
wrist, saying, ‘You built an article that broke the page
size budget! You have to take that down or change that
immediately!’
“Our goal setting out was to set up best practices, make
recommendations, and be a resource within the
company that people can turn to when they have to
make performance-related decisions.”
Dan Chilton, Vox Media
responsivewebdesign.com/podcast/vox-media-performance/

“We, as engineers,
should learn how
to show the impact on
anything we do.”
Malek Hakim // Priceline
chasingwaterfalls.io/episodes/episode-one-with-malek-hakim/

How often is often enough?
Wall monitors and dashboards 24/7
Alerts (to people who can make fixes) in realtime
Reports no more than 1X week
Meetups, hackathons, etc. monthly (if possible)

7. Don’t forget to celebrate!

medium.com/the-telegraph-engineering

“The dull boring stuff”
~Andy Davies
Scripts (especially third parties)
Images
Extraneous code
Defer assets where possible

Shaved 15KB off logo
Ran A/B test
Increased bookings
chasingwaterfalls.io/episodes/episode-one-with-malek-hakim/

There’s no magic.
Show up with a plan.
Do the work.
(Be patient.)

Thanks!
@tameverts
speedcurve.com/blog

2020 Chrome Dev Summit: Web Performance 101

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