There seems to have been some good work being done on the GC.

* Project lead(?) claiming 1ms pause times: https://www.reddit.com/r/ocaml/comments/7yqa92/comment/dujyj...

* General status update: https://www.reddit.com/r/ocaml/comments/7yqa92/comment/dujgx...

Iirc the flambda work had to be integrated with the multicore branch. That must have been some serious effort.