An opensource framework allows interoperable and inter-organizational use between different offerings (or servers). Signal can't be compared in this category, because it is simply an opensource client offered by one provider or organization, and not a standard for interoperable use: it can be...
How is SIMPLE for an XMPP messaging alternative?
Both are accepted IETF open-source standards.
XMPP is great, except it has a lot of overhead (sometimes heavy lag) because it uses XML. Is SIMPLE as secure as XMPP?