Intel is marketing the Optane Memory M.2 modules as caches for hard drives.
"Lather, rinse, repeat. With each duplicate task, the launching speed accelerated. The load time for Gimp, for example, dropped from about 14 seconds to 8 seconds, and then to 3 or 4 seconds as the Optane Memory cached the task."
That's only speeding up accesses for repeated tasks (which, granted, there are many of).
I think the problem Intel found is that Optane memory is too expensive right now in larger sizes. They came up with this cache module as their best way to market it. Is someone really going to spent $77 for a 32GB cache device when they can just spend $99 for a 256GB SSD?