HDX is a system that includes the interface as part of the equation. It is as if, you have the power of a large computer, inside your large computer, running Pro Tools and its processing. HDX gives you a direct connection to Avid HD Interfaces, up to 64 I/o per card, for bullet proof integration with the software's frame. It has extremely low latency and the Avid interfaces also have extremely low residual latency. The HDX card has a Cache of DSP Accelerated power for the Pro Tools Environment and background tasks are handled by the DSP on the card, and not your computer. It also has a Cache of DSP for AAX-DSP Plug ins. It is a Scalable Core Card System, and as you add more HDX cards, you add more DSP Power, and more I/O capabilities with HD Interfacing.
I think only Protools HDX has ultra low latency with fx on input channels. Protools HDX has a bussing system from the input to the processing units to the outputs.
All other solutions are compromises in some way. Normally some audio gets routed to the Intel processor and that's the bottleneck. Intel never built those system designs for pro audio purposes, unfortunately.
Apple should talk to Intel and improve this situation fundamentally. Every iTV needs 0.7 ms analog to analog processing with FX in the near future ;-)