Sorry for the negative vibe, I guess with an annotation you could keep single threaded as the norm and add @Parallel to components which are slow.
Under the hood, all rendering is done via the RenderCommand interface which could, in theory, be parallelized. You'd probably have a pool of render threads for the parallel work. These threads would need to be initialized and cleaned up and be able to access per-thread values (including Environmentals and services etc). You could probably record() any thread local actions on the request thread and replay() them on the worker thread. You'd need to consider ordering in cases when one component needs to render after another (think @Environmental dependencies or DOM dependencies). Since a thread-safe DOM will be slower, you'll want to check for @Parallel and default to the faster DOM if no @Parallel is present.
