How much does site speed affect sales?

Enough to be worth a week of work and not enough to save a store that has other problems. The clearest effect is on paid traffic, where a slow first screen loses people who never chose to visit.

Why the answer is what it is

The measurement worth making is time to the first useful screen on a mid-range phone on mobile data — not a score on a desktop test. Images are almost always the cause, and almost always the cheapest fix.

What this looks like in practice

Discounting to grow trains customers to wait for discounts. It works once and costs afterwards.

Traffic is the expensive problem and conversion is the cheap one. Doubling a 1% conversion rate is usually easier than doubling visitors, and costs nothing per month.

Retention beats acquisition on every measure, and almost every store spends the other way round.

Where a mobile app comes into it

Push notifications are the one growth channel you do not rent. Every other route to a customer is paid for again each time.