I’ll call this a “final” note on this discussion.
The foxy-future branch is now officially merged and ready for use.
See the README for some info about it.
The branch is also mentioned in the foxy branch README, as well as in the Foxy recording/playback tutorial
We now have open a list of targeted PRs that we would like to try and backport to Foxy. Backport performance improvements to Foxy · Issue #657 · ros2/rosbag2 · GitHub lays out this list. Some may be fairly difficult and need significant rewrite to work on Foxy - this is acceptable for a non-breaking improvement. Please consider this post an open call-for-help for those who have bandwidth to attempt these backports. Note that API and ABI breakages will not be considered for release into Foxy.
Cheers!