Subspace Ansible: The tangle channel, which involves manufactured entangled (not in the standard quantum sense, note, because we know that doesn’t work; these are ontotechnological devices using the “privileged channels” a long way behind those) particle-pairs. This makes them quite expensive (since they are a consumable resource, one particle per bit transmitted, and have to be shipped there the long way once you separate the ends; if you don’t have one or a stargate, your best option is a lighthugging communications torpedo) at least relative to using light-speed EM communications and relaying them through the stargates, the way most of the non-priority extranet works, but they’re invaluable for priority communications and beyond the reach of the stargate plexus. (They are, for example, the only means of ready communication available to lighthuggers.) And yes, they do work for mindcasting.
(And, yes, they can also let you play interesting games with causality. Just as expected.)
That said, extensive use of caching, prefetching, and AI traffic prognostication makes the extranet delays mostly invisible in practice, as does the ability to engage in pseudo-real-time communication by sending a partial copy of you along with, or as, your message to be able to have a real discussion with the recipient, then reabsorb it when it returns.