FPS Direct Participant membership is a very closed circle. Transferwise just got their membership a few weeks ago, and they were founded two years before .
Until then, challenger banks must route their FPS through direct participants, and get charged in the process.
I believe it is not cost-effective* to send and receive FPS in real-time because of that, so they must be subscribing to periodic batches.
On top of that, the mandated Direct Participant probably runs additional AML checks on these routed FPS since they haven’t KYC’d the counter-party themselves.
(*One could argue that Premium and Business customers should have priority access. Or to rephrase: what do we pay for “turbo” transfers then?)