It is a really bad idea to compromise security for speed and connection stability. Processing power is a question of scaling the hardware, communication speed is hardly affected by a proper encryption scheme, neither is reliability of the application-layer protocol.

Usability, however, is a different beast. You must compromise security to make a chat application appeal to "regular" users. Still, this is a trade-off that can be clearly communicated to the user and to developers, and does not require a custom crypto protocol. You can achieve the same effects with using existing and tested libraries.

Edit: Disclaimer: I am the developer of the yaxim Android XMPP client and the operator of the public yax.im XMPP server (both available at http://yax.im).