This idea that, by remaining ipv4 and letting other services make the move, somehow makes your ipv4 service faster/less congested is entirely bogus.
Who is using physical instances to provide any of these hops in question? It is logical interfaces on the same physical infrastructure. You have a choice: use NAPT44 or gonwith ipv6 and no NAT whatsoever. Facebook are saying the latter gives better performance. Remove a layer4 service mid hop and your end to end performance improves, seems plausible.