In my experience, still not working well.
I frequently find that after a few edits (particularly clusters of flags, ratings, or keywords), that everything wedges. Things like clicking the Pick flag no longer work (the flag stays unset in the grid or loupe) - in fact, they work, but are queued behind something that could easily take an hour or two to finish. It appear to be a sync issue, because once it has caught up, pausing sync seems to make the problem go away. But the after turning on sync, it's easily stuck -- usually with a whole set of images in LR "downloading develop data" - even though I almost never edit in LR.
I also find that it's frequent that when I'm editing a single photo, that viewing a different photo and returning to the edited photo often appears to have "lost" the edits I've just performed. What appear to be happening is that the edits are synced up to Lr and then synced back to LrC, and it's not uncommon to watch the History panel in LrC revert to empty, then gradually fill in with the edits performed 5 or 10 minutes previously, until with a long lag, the edits are all "restored".
This superficially manifests as a giant performance issue; I'm convinced it's actually a sync issue.
My master library on LrC is 330k photographs, of which about 100k are synced to LR, where I do occasional editing, but never loading new originals (maybe a total of 10 pictures from a mobile device edited directly on that device.)