For a CMS or any similar situation, you can get huge performance improvements from higher level changes than Span<T>. Using the HTTP cache-control headers correctly in conjunction with a CDN can provide an order of magnitude improvement. Simply sending less HTML/CSS/JS by using a more efficient layout template can similarly have a multiplier effect on the entire site.

In my experience, the biggest wins by far were achieved by using the network tab of the browser F12 tools. The next biggest was Azure Application Insights profiler running in production. Look at the top ten most expensive database queries and tune them to death.

The use of Span<T> and the like is much more important for the authors of shared libraries more than "end users" writing a web app. Speaking of which, you can increase your usage of it by simply updating your NuGet package versions, .NET framework version to 9 or 10, etc... This will provide thousands of such micro optimisations for very little effort!