Before you invest in mobile performance

It is rarely the thing that gets a project approved, and often the thing that decides how it goes. Before you spend anything on mobile performance, it is worth confirming a few things are already true.

Performance work is mostly subtraction, which makes it unpopular and effective. The practical test is whether someone new to the project could tell, in a minute, that it had been handled.

Prerequisites

• You can describe the outcome you want in one sentence
• Someone owns it after the work is done
• Phones have less processing power and worse networks
• You have a way to tell whether it worked

What to watch for

Test on mid-range devices, not just the newest. It is worth being explicit about, because assumptions differ quietly. The failure mode is not doing it wrong, it is doing it once and assuming it stays done.

JavaScript costs far more on mobile than on desktop. Small and consistent beats large and occasional here. If it only works because one person remembers to do something, it does not work yet.

The short version

Speed is a feature people notice only in its absence, and then they leave rather than complain. Three things worth confirming about mobile performance before you move on:

• Someone can say what the current setup is without going to look
• Phones have less processing power and worse networks — and you know whether that is true here
• There is a way to tell whether the last change to this helped

Most of the value here comes from doing the first two things, not all of them.