Don't trust your desktop score: why mobile speed is different
PageSpeed gives you 95 on desktop and 42 on mobile. The two tests simulate very different conditions, so the gap is expected. Here is which one to pay attention to.
You test the same page. Desktop says 95, mobile says 42. The first reaction is usually "the test is broken", but the test is fine.
The two tests measure different things
The mobile test deliberately simulates tough conditions: a mid-range Android phone on a throttled 4G connection. The desktop test assumes a fast computer on broadband.
A low mobile score doesn't mean your site is broken. It shows how the site behaves in poor conditions, and that's exactly where most of your visitors are.
Which one matters?
Mobile, for two reasons.
First, Google ranks sites using their mobile version (mobile-first indexing). Even if your desktop site is excellent, the assessment is made on mobile.
Second, most web traffic now arrives on phones. Check your own Analytics report and you'll probably see a figure above 70%.
What breaks most on mobile?
The cost of JavaScript
A phone's processor is slower than a desktop one. A script that runs in 200 milliseconds on a desktop can take 1.5 seconds on a mid-range phone, which is why heavy JavaScript hurts mobile scores the most.
Serving images at one size
If the 2400-pixel hero image made for desktop is sent at the same size to a 390-pixel phone screen, the mobile visitor downloads six times more data than needed. Responsive images (srcset) fix this.
Tap targets and horizontal scrolling
These aren't about speed, but they show up in the mobile usability report: links too close together, tables that don't fit the screen, text smaller than 16 pixels. Google measures these too.
Where to start
Turn off wifi on your own phone and open your site on mobile data. If the wait annoys you, it annoys your visitors too, and they're less patient than you.
Then look at the "Opportunities" list on the mobile tab in PageSpeed. Google tells you how many milliseconds each item will save. On most sites, fixing the top three is enough.