bug#19776: 25.0.50; HTML rendering is very slow

[Stefan Monnier]

> But this would be the first special form that doesn't have a byte op
> code, probably?  Is that allowed?  I couldn't find any other examples of
> that being a thing.

You don't need a new byte-code, but I'd strongly advise against using
a new special form.  It'll be much simpler to define it as a macro +
a function, where the macro turns

    (with-delayed-message MSG BODY)

into

    (funcall-with-delayed-message MSG (lambda () BODY))


-- Stefan