Can my agency/developers make a difference?

As frontend architecture and thus HTML, CSS and JS are responsible for the work a browser has to do, this very same frontend code will have its influence on the metrics. As a result, your developers are the ones that will have to do the coding, where CRO or SEO specialist are often the ones pushing for optimization work.

Obviously, one platform will allow more room for improvements than other platforms, but your developers should still be capable of optimizing for improved Core Web Vitals and conversion.