Ask for performance evidence, not a portfolio
A portfolio shows what a company built. It does not show whether those sites load fast or rank. Ask for a Lighthouse score, a Core Web Vitals reading or a live URL you can test yourself.
The public Core Web Vitals thresholds are LCP under 2.5 seconds, INP under 200 milliseconds and CLS under 0.1. A site that clears those numbers respects the visitor and the search engine. A company that cannot show you a number is showing you a promise.