Your Next.js site is fast on your laptop and slow on your customer's phone

Every performance conversation we have starts the same way: someone opens the site on a MacBook over office fibre, sees it paint instantly, and concludes there is no problem. Then we open it on a three-year-old Android on a shared mobile connection and the same page takes nine seconds.

The device is the bottleneck, not the network

Bandwidth in Dhaka is fine. What is not fine is the CPU on the median device parsing 900KB of JavaScript. A modern phone chews through that in 300ms; a mid-range one takes 2.5 seconds, and it does it before anything on screen becomes clickable.

Three changes that move the number

• Server-render anything that carries content. If a route exists for SEO, it should not need JavaScript to show text.
• Budget your JS per route, not per site. A homepage carrying the checkout bundle is the most common cause we find.
• Test on a throttled profile in CI. If the regression is not caught by a pipeline it will be caught by a customer.

What we do on every build

We agree a Core Web Vitals budget in week one and fail the build when a branch breaks it. It is an unpopular meeting and it saves the project.