No plugin, service, or magical bit of coding can make up for great content. When it comes to SEO, what's most important is creating high-quality, trustworthy, and useful content.
But even the best content can suffer when its poorly delivered. For several years Google has used page load times as a factor in search engine ranking, giving higher priority to websites that deliver content to users fast.
Optimizing WordPress for speed used to involve complex caching software, setting up refresh rules, and hand-crafting code to be as compact and quick-to-deliver as possible. But now, thanks to services like Cloudflare, staying up-to-date with the latest best practices is much, much easier.