Comment 44 for bug 1306743

Most of the optimisations haven't landed, and some of them involve internal API changes so they may not be appropriate to backport.

(I think they can be backported as API additions with non-deprecated shims)

https://review.openstack.org/#/q/status:open+project:openstack/heat+branch:stable/icehouse+topic:bug/1306743,n,z