Looks good so far, but:
.. even the default keystone theme is ultra bad optimized..
The Yaga plugin works ONLY on fresh installed Vanilla without any activitys on it like (comments, posts and so on..)
If you try to install yaga plugin on the forum after u have posted any threads, it will not work, the same bug will occure like u have posted it already on your new forum theme..
I moved this out of the other thread because it's a bit unrelated. I'll note we'll have the 2.8.1 release out soon which will finally have the feature flag for deferred scripts ready for production use. I found this to greatly increase the page speed score during testing. (brings it up to ~45 on mobile and ~86 on desktop) for me.