Some work on asynchronous block processing may be going into the PHP RunKit extensions. However I am not yet certain how shared memory is handled, if memory is shared in the normal way. It could be problematic if it is not automatic.
Naively trying to do stuff in parallel, but having separate runtime caches and execution contexts in each thread, could be a bad idea.

Summary: This tracker issue is very much an unknown at this point.

More trouble than its worth. The self-learning cache has already made performance great, we'd be doing enormous bookkeeping (lots of extra code) for a really tiny gain. The extra code could easily cause more resource usage than that gain.