tl;dr web-components are only slow if they have a shadowRoot.
My intention with the upcoming version of b8r is to replace b8r’s components with web-components, and my previous test showed that web-components were kind of expensive. But it occurred to me I hadn’t compared them to b8r components, so I did.
In a nut, a b8r component is about as expensive as a web-component with a shadowRoot (the b8r component in question was styled; removing the style didn’t improve performance, which isn’t surprising since b8r deals with component styles in a very efficient way), and a web-component without a shadowRoot is just plain fast. This is great news since it means that switching from b8r components (which do not have a shadow DOM) to web-components is a perf win.