Written by on CSS Wizardry.

Table of Contents
  1. Why iOS Gets Overlooked
  2. Why This Is a Problem
    1. Why This Isn’t a Huge Problem
  3. But, Can’t I Just Emulate?
  4. You Will Need…
  5. Testing with WebPageTest
  6. Testing in Safari’s DevTools
    1. Set Your iPhone up as a Development Device
      1. Network Link Conditioner
    2. Enable Web Inspector
    3. Inspect Your Phone From Your Mac
  7. Why Bother?

So far this year, all but one of my clients have been concerned about Google’s
upcoming Web Vitals
The client who’s bucking the trend is great, not least because it’s given me
something a little different to focus on—they’re more interested in how their
site fares on iOS. What makes this particularly fun for me is that iOS Safari is
a completely different ballgame to Chrome, and not something many people tend to
focus on. So, I’m going to share with you a handful of tips to make it a little
easier should you need to do the same—and you should do the same.

Why iOS Gets Overlooked

Google has a pretty tight grip on the tech industry: it makes by far the most
popular browser with the best DevTools, and the most popular search engine,
which means that web developers spend most of their time in Chrome, most of
their visitors are in Chrome, and a lot of their search traffic will be coming
from Google. Everything is very Google centric.

This, of course, is exacerbated by the new Vitals announcement, whereby data
from the Chrome User eXperience Report will be used to aid and influence

In short, it’s easy to see why Safari gets left out in the cold.

Why This Is a Problem

The moment you don’t consider Safari, you’re turning your back on all of your
iOS traffic. All of it. Every browser available on iOS is simply a wrapper
around Safari. Chrome for iOS? It’s Safari with your Chrome bookmarks. Every bit
of iOS traffic is Safari traffic.

In short, it’s difficult to see why Safari gets left out in the cold.

This isn’t Google’s fault, and I’ve long wished that Apple would let other
browsers on their platform, but that doesn’t seem likely to happen any time
soon. So, we’re stuck only with Safari. That we probably aren’t testing.

Why This Isn’t a Huge Problem

It’s worth noting that, by and large, the same page will perform better in iOS
Safari than it would on Android Chrome—iPhones are generally far more
than their Android
counterparts. Further, and by chance, iOS
strongly correlated with regions we generally find to have better
. This means that iOS devices tend
to be faster and are found in ‘faster regions’.

Measuring Network Performance In Mobile Safari
Same page, same connection speed, same locale, same release year, different devices.

But, Can’t I Just Emulate?


You Will Need…

For better or worse, I’m an iPhone and Mac user, so I’m pretty well set up out
of the gate. If you don’t have an iPhone, well, you’ll struggle to test an
iPhone. But the additional need for a Mac will be a barrier to entry for many
I’m afraid.

But! Fear not! It’s not the end of the line for non-Mac users. Read on.

Testing with WebPageTest

WebPageTest is easily, by far, without a shadow of
a doubt, the single most important tool when it comes to web performance.
I could not do my job without it. I cannot overstate its importance. It’s vital.

So imagine how happy I am to report that it has first-class, genuine iOS device

Measuring Network Performance In Mobile Safari
Real iOS devices—unreal!

This will give you a fantastically detailed
plenty more—that you can go back and refer to time and again.

However, there are two key caveats:

  1. The devices are in the US which doesn’t help if your typical customer is
    based in, say, the Balkans.
  2. The data is static which isn’t necessarily a drawback until it
    is—DevTools allows us to work far more quickly and interactively, responding
    to changes and tweaks as we make them.

But still, this is an amazing starting point for anyone wanting to start
profiling web performance on iOS.

What we really want to do, alongside capturing good benchmark- and more
permanent data with WebPageTest, is interact with and inspect a site slightly
more realtime. Thankfully, if you have a Mac and an iPhone, this is remarkably

Set Your iPhone up as a Development Device

N.B. This step and its sub-step are optional, but highly recommended.

Plug your iPhone into your Mac and fire up Xcode. Once its opened, go to
Window » Devices and Simulators and look for your device in the resulting

Measuring Network Performance In Mobile Safari

Make sure that any option or alert that pertains to running your iPhone as
a development device is enabled. Truthfully, I’ve never really had to do
anything more in this view than unlock my phone and maybe restart it once or
twice. As long as I see this much, I can guarantee the next step will work…

I honestly can’t even believe I’m telling you this… this is so cool. Now, in
iOS’ Settings app, you should find a new Developer option.

Measuring Network Performance In Mobile Safari

In there, you should find a tool called Network Link Conditioner. This
provides us with very accurate network throttling via a number of handy presets,
or you can configure your own. This means that, even if you’re connected to the
office wifi, you can still simulate slower (and very realistic) connection

Enable Web Inspector

Next, in Safari’s settings on iOS, head to Advanced and enable Web
. That’s it! You’re good to go.

Measuring Network Performance In Mobile Safari

This will allow your desktop version of Safari to inspect the current tab of
your iPhone’s Safari instance.

Inspect Your Phone From Your Mac

Now open Safari. I know, you haven’t opened it in a while, so hit
⌘⎵ and type Safari. Head to Develop and look out for your
device in the dropdown menu.

Measuring Network Performance In Mobile Safari

…and that’s it. That’s really it. You’re now inspecting your phone from Safari’s
desktop DevTools:

Measuring Network Performance In Mobile Safari

As long as your phone stays unlocked and with the webpage in question active,
you can inspect the iOS device as you would desktop. Neat!

Of course, the whole point of this article is performance profiling, so let’s
move over to the Network tab. If you’re used to Chrome’s Network tab then,
well, I’m sorry.

Measuring Network Performance In Mobile Safari
Regrettably, this far into the article and its screenshots,
I learned that there is no longer a way to disable ServiceWorkers in Safari.
This means none of the data in the screenshots is particularly interesting, but
thankfully we aren’t here to profile my site.

Here is how my actual iPhone on a throttled connection deals with my website.
There are some truly fascinating differences between how Chrome and Safari work,
visible even on a scale as small as this. It really helps drive home the
importance of testing each platform in its own right, and explains why you can’t
simply emulate an iPhone in Chrome’s DevTools.

But! That said. Chrome’s DevTools are far, far better than Safari’s, so we might
as well use that to our advantage, right?

Export the HAR (Http ARchive) file from Safari’s Network panel and save it
on your desktop where it will remain until you buy a new machine
somewhere sensible. Next, open Chrome and its own Network DevTools panel.
Import the HAR file here.

Measuring Network Performance In Mobile Safari
Viewing Safari’s Network data in Chrome’s superior DevTools.

This is key: Safari captures less data than Chrome, which is a problem. Simply
opening this file in Chrome’s DevTools doesn’t make the data itself better, but
it does allow us to interrogate it far better. It’s basically just a far nicer
workflow onto the data provided by Safari.

If you want to know more about how best to use DevTools for performance
testing, I’m running a workshop with Smashing Magazine real soon, and that will
make you DevTools experts. You can still grab

Why Bother?

With iOS generally being faster, and Vitals being entirely Chrome centric, and
most emerging economies being Android-led, it might seem redundant to test
performance at all on iOS. In fact, it’s a relatively safe assumption that if
you’re fast on, say, a Moto G4, you’ll be blazing fast on an iPhone. But still,
given the sheer prevalence of iOS, it’s wise to be aware of what might currently
be a total blind-spot.

While it’s unlikely to become your default process, it’s important to know how
to do it. And now you do.

Still worried about Vitals? Drop me a line! There’s still a little
while before the May update.

☕️ Did this help? Buy me a coffee!


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