Ask what they'll measure, and where. A good specialist starts from field data: Core Web Vitals from real visitors, as reported in Google Search Console or the real-user section of PageSpeed Insights. A proposal built around moving a lab score from 40 to 90 is a proposal to pass a test, not to speed up your store.
Expect a cost for every app and tag. The audit should list each app and third-party tag that loads on the storefront, how much script it adds, and whether anyone still uses it. It's common to find an app nobody on the team can explain, still loading on every page.
Check which templates they'll test. Paid and organic traffic usually lands on product and collection pages, and those are often heavier than the homepage. Ask for results per template, tested on a mid-range phone over mobile data, because that's the shopper who notices a slow page first.
Ask how they'll keep the apps working. Deferring a script is easy; deferring it without breaking reviews, subscriptions or tracking takes care. The specialist should work on a duplicate theme, say which changes need your sign-off, and hand over a list of every change made.
Red flags. Guaranteed scores. A "speed booster" app offered as the fix, because adding an app rarely solves an app problem. Lazy-loading everything, including the hero image, which makes the main loading metric worse. An audit that still reports First Input Delay, which INP replaced in 2024. And a recommendation to change themes before anyone has measured the current one.
Agree the handover. Speed decays as apps and campaigns get added. Ask for the baseline numbers, the templates tested and a short list of what to check after each new install, so your team can spot a regression without paying for another audit.