Those nodes in the queue have to come from somewhere and that is where the inefficiency lies.

If you use SIMD gather instructions to fetch all the children of a node, that is still going to be very inefficient on e.g. a Xeon. Each of those gather loads are likely to hit different cache lines, which are accessed only once and waste 7/8th of the memory bandwidth pulling them in. SIMD is also not going to help if you are memory bandwidth or latency bound, we're talking ~0.1 ops/byte here.