First contentful paint (FCP) is basically the same (+-10ms) from React down to a raw HTMLElement. I guess at some point appending elements to the DOM is the bottleneck, rather than the overhead of parsing JS and the efficiency of said JS.

To get a faster FCP, statically rendering the component to HTML and re-hydrating it in the client (using Gatsby, for example) seems like it will have a bigger impact than the WebComponent library used. Unless the client is bandwidth constrained of course.