Does shadow DOM improve style performance?
This article is about style performance improvement with Shadow DOM, but it also contains a useful comparison of selector performance between ids, classes and data attributes:
ID and class selectors are fast enough that actually it doesn’t matter much whether shadow DOM is used or not – in fact, they’re slightly faster without shadow DOM. This indicates that systems like Svelte, CSS Modules, or good old-fashioned BEM are using the best approach performance-wise.
This also indicates that using attributes for style encapsulation does not scale well compared to classes. So perhaps scoping systems like Vue would be better off switching to classes.