Also, a DID needs a resolver that can find information about you based on the DID alone. With the keypair example you're likely hitting MIT's keyserver to try to resolve it, or Keybase or whatever.

The good thing about ActivityPub IDs is that you can easily resolve them from the ID alone, since they're URLs. Most other solutions have us depending on centralized servers, or reinventing DNS. IPFS seems like it would work well for this, if it wasn't extremely slow.