I found the issue of any installation using any English language other than English en_US being able to benefit very interesting as it had not been immediately obvious to me, until I thought about it. The main issue is not translations of the content, but that of the language of the WordPress installation.
Most of my sites are single language, but use English en_GB and when optimised they are fast enough, but with the Performant Translations plugin they definitely load faster. This would also apply single language sites using en_AU, en_CA, en_NZ, and en_SA, not to to mention all the other languages.
I was particularly interested in the plugin, because I have one trilingual site with many large images and videos, which is sluggish especially on the backend. The page loading time was reduced although the backend is still slow.
I believe the case for integrating the plugin into the core is well proven and I hope it will be implemented as soon as possible.
