If you're only checking your website speed on your office Wi-Fi and MacBook Pro, you're living in a fantasy. 70% of your users are on mobile, and they are suffering.
1. The Vanity Metric: Desktop Scores
Scoring 99/100 on Desktop is easy. You have a powerful processor and a fiber-optic connection. But your customers are on 4G LTE, in a coffee shop, with a 3-year-old Android phone.
Google uses Mobile-First Indexing. This means they grade your homework based on how your site performs on a low-end mobile device, not your high-end desktop.
Network Constraints
Mobile networks have higher latency. A 2MB image that loads instantly on desktop might take 4 seconds on mobile. That 4 seconds is enough to kill the sale. We call this the "Mobile Tax"—the revenue you lose because you didn't optimize for the weakest link.
2. Treatment Plan
Respect the user's bandwidth. Resize images specifically for mobile screens (using `srcset`). Defer non-critical JavaScript. If your mobile score is Red (<50), you are largely invisible to Google.