A slow, frustrating checkout that causes abandonment affects session quality signals and return-visit likelihood, both of which search engines account for indirectly through engagement and branded search behavior over time.
A bad checkout experience generates negative reviews and complaints that get indexed on third-party sites and show up in branded searches — directly shaping what people see when they search your store name before buying.
Treating checkout speed as a revenue-and-reputation metric rather than a pure UX nicety tends to get it prioritized correctly — it compounds into exactly the kind of brand and engagement signals that indirectly support everything else you're doing for SEO.