[drewcrawford]

> You could keep some of these optimizations if Swift's message-passing mechanism preserved type information.

Type information isn't what makes this fast. What makes it fast is the very guarantee that dynamic dispatch breaks: knowing, for a certainty, where the function will land.

> As I mentioned, app extensions may be a step towards a more workable solution, but today, the ability to add PGP to mail using method swizzling is important to thousands of Mail.app users.

A fast Mail app is important to millions of Mail users.

If PGP is important then one interim possibility is to write your own mail client. That requires extra work, but one of the hallmarks of important problems is that we are willing to do extra work to solve them.