What a slow checkout actually costs

The most expensive traffic you have

A visitor at checkout has passed every earlier filter. If they came from paid media, you have already paid for them in full. Losing them at the final step wastes the entire acquisition cost, which is why latency here costs more per second than anywhere else on the site.

Yet cart and checkout typically carry the largest script payload — payment providers, upsell apps, analytics, chat widgets, review tools, each added at a different time by a different person for a good reason.

Why mobile is where it happens

Most paid social traffic arrives on a mid-range phone over mobile data. That device parses and executes JavaScript far more slowly than the laptop the store was tested on, and it does so on a connection with meaningfully higher latency.

The result is that a checkout which feels instant in QA takes several seconds in the hands of the people actually buying — and nobody internally ever experiences it.

What the fix looks like

Defer or remove non-essential scripts from cart and checkout specifically, rather than site-wide. Inline the critical CSS the payment path needs. Surface shipping and tax earlier so the final step holds no surprises. Then test the whole path on a real mid-range device over throttled data, because that is the only test that reflects reality.