A built-in parallel foreach can't guarantee better performance because the hardware isn't there. The parallel part is scheduled on normal threads by the kernel and this is too unpredictable.

But if each OS-scheduled CPU was really a cluster of for instance 1 fast + 64 slow cores then it could have special instructions to split a low-level loop on these slow cores and to facilitate blocking and rejoining back into a single stream. A 'parallel branch' instruction could take index, limit, and estimated instruction count and the CPU itself could decide whether to run it in a single fast stream or in parallel on slow cores.

There's some technical challenges like the size of a register file (so probably uninterruptible), but this may be where we're going since there's diminishing returns from making a single CPU faster and there's lots of opportunity for parallel processing that GPUs are too clumsy for.

That's pretty much the usual GPU+CPU workflow... Only you would have less badwith and latency issues.

Still, deciding where to run such things are not trivial, the time it takes to decide what would be faster could actually be more than the time it takes to just run it.