The e2e crypto does use AES-256. We have been working to keep the impact to a minimum. It is completely invisible for the typical message or PPT upload.

On the interop question: the best way to interop with someone who doesnt use it, is to just have them use it. You can invite anyone with a valid email address and they can get the app and use it to communicate with you, or anyone else, for free. We are also supporting SIP and XMPP.