A few things I would like to further discuss and potentially change:

1. Preloading language. Edit: Done.
Both plugins know which language the code is before the file is received. This can be used to preload the language definition using the Autoloader which has to fetch it anyway.
This requires exposing Autoloader's language loading API (already done in #1898). Alternatively, we could also create a fake element to highlight which will cause Autoloader to load the languages.

2. Fixing callbacks. Edit: The fix in #1973 is good enough
Right now, the callback function for the pres won't be executed at all. #1973 fixes this but only calls it synchronously.
To fix this I would like to add a new hook: delegate. The purpose of the hook is to allow highlightElement to hand off the responsibility of highlighting the given element to some other instance. All registered hook callbacks will then be asked whether they take the element and the first to say yes has the responsibility to handle highlighting the element (including calling the callback).
The environment of the hook I imagine like this:

interface DelegateHookEnvironment {
    element: Element;
    async: boolean;
    callback?: (element: Element) => void;
    language: string;
    delegate: boolean;
}

Hook callbacks will set the delegate property to true to say yes.
The hooks will be run before any DOM mutation, so here and the code to do so could look like this:

const env = { element, async, callback, language, delegate: false };
for (const f of Prism.hooks.all['delegate']) {
    f(env);
    if (env.delegate) return;
}

Thoughts?

3. Asynchronous highlighting. Edit: Maybe later.
Right now, both FH & JH ignore the async parameter and highlight code synchronously.
Note: The proposed solution for 2) fixes this issue as well.