Website performance is a key goal towards e-commerce conversions, so it must be
a core priority for business stakeholders and leadership, not just development
For that reason, it’s crucial to make performance metrics visible and tangible
for all stakeholders, and to build reporting and monitoring into your
This guide describes what to be aware of and what to avoid to achieve
A word about metrics
There are countless metrics to evaluate website performance, and while it may
seem useful to collect all of them, too many metrics can be confusing and
misleading. There are several ways to deal with this:
- Gather multiple metrics and try to narrow and filter afterwards on
what might be relevant for the task at hand.
- Abstract metrics into an overall score, as for example
This can be especially useful for non-technical staff and other
stakeholders, but is probably insufficient for deeper technical analysis.
- Try to find the one metric which is most relevant as a predictor for
your conversions and then optimize towards this.
In reality, a pragmatic combination makes the most sense here. Gather more rather
than less to begin with, report an abstract score to management, and optimize
towards the one metric which best predicts your conversions.
Finding this metric can be done by using your analytics tool of choice
to map performance metrics to user engagement, conversions and transaction values.
For example, a custom report to do so looks like this in Google Analytics:
Metrics can be deceiving.
Unfortunately metrics can sometimes be misleading with respect to performance.
Keep reading for specifics.
Increasing bounce rate
Often it is assumed that bounce rate will drop when page load speed
increases. While this normally does hold true, measurements sometimes show the
opposite. This is because analytics can only measure a bounce after the
analytics library is loaded. A faster page load means analytics code also loads
faster, so analytics may see more bounces even if there aren’t more happening.
This can be eased by measuring
real page abandonment instead.
Decreasing relative conversions
Relative conversions may sometimes seem to drop for faster sites.
This is because faster pages reach a bigger audience who might be less engaged
or committed. While incremental traffic and conversions increase with faster
pages, relative conversions (the ratio of conversions to page views or visitors)
might still drop.
This effect can be eased by looking out for absolute conversions instead, and
even calculate Cost Per Sales (conversions divided with investment level) or ROI.
Page engagement may seem to drop for a faster page.
This can actually be a positive signal! If the page is faster
users can reach their goal more quickly and might just have shorter sessions and
less time on page.
In general, it can be deceiving to look at averages and medians of
performance metrics (see this nicely described in
Everything You Know About Latency Is Wrong).
Similar to a chain being just as strong as the weakest link, the performance of
a funnel is only as good as its slowest load. A single slow load may be enough
to lose the user. Therefore averages and medians are more likely to hide the
real performance issues, than to reveal them.
It’s best to analyze complete distributions, but as this is not
always easily possible we recommend using 90th percentiles—this is the value
for which 90% of loads were faster. Even then a user hitting ten pages will
still have one load slower than this, and might drop out there.
While performance measurement is highly important, make sure to keep an open
mind and question unexpected results—and make sure to not report misleading
numbers to stakeholders and management. If unsure on what to pick and report,
we’d advise as a minimum for 90th percentile
First Contentful Paint, which is also what we use
across our public tooling.
Third party content
Website performance is especially prone to being dragged down
by third party content
(see Eliminate render-blocking resources).
This is a particular problem for e-commerce, often due to trackers and widgets.
Some ways to handle third party content with respect to performance:
- Always keep third party content out of the
critical rendering path.
If the third party has server problems and times out, it will impact your
website heavily. You can test and simulate this via
- Continually measure and report the ratio of third party content, for
example via WebPageTest domain
breakdown. Make sure to use
performance budgets for third party content.
- If you suspect that a particular third party component is having a
detrimental effect on performance, do a performance comparison with it
included and excluded.
Find out how.
- Try to stay on one stack from one vendor if possible. For example, if
you have a tag manager and analytics on one stack, you may only need a
single script, and may be able to
take advantage of HTTP2 synergies
as there is only one host involved.
- Make sure not to use the same functionality from two different vendors.
You shouldn’t need two tag managers or two analytics platforms.
- Routinely audit and clean out redundant third party scripts, trackers
and widgets. This can easily be done via Ghostery Extension or tools like WhatRuns:
Learn more in
Unfortunately performance is often seen as a one-off optimization task, and then
regresses over time as stakeholders raise new feature requests or insist on
adding new trackers and widgets.
Performance must be a continuous goal to improve acquisition, discovery, and
conversion rates as well as safeguarding the reputation of your brand. This can
be achieved with performance
like Tinder did,
and by establishing and fostering a performance culture, where all employees and
especially decision makers recognize speed as a core feature of the website.
Make metrics tangible
Reports on metrics are often abstract and easy to challenge or dismiss. It’s
best to make your performance tangible and visible. There are several good ways
to do this:
and Google do this by providing slow networks across the company for testing.
- Make average, low-spec devices with low
bandwidth or high latencies available
to management and other stakeholders.
- Consider adding overlays showing performance metrics on your development
or staging servers. Connections to these servers from mobile can
potentially be throttled by default on corporate networks.
- Monitors can be placed strategically across the company showing videos
or timestrips of your website’s loading behavior, preferably in comparison
to competitors. WebPageTest can create
these very easily and automatically.
- Performance can also be fun—maybe a game based on an individual Lighthouse report can reach audiences which can’t be reached by pure reports and metrics?
This guide explains why the selection of metrics, and how they are reported and handled, is as important as the measurement and optimization itself—or even more. Make sure to prefer percentiles or distributions over averages, be cautious of using just bounce rate or relative conversion rate as impact measures, and make sure that metrics are easy to understand and tangible for stakeholders across the company. Establishing a performance culture is also an important step towards a well performing e-commerce site.