I have to give credit where credit is due, the team behind the PC version of Ys IX: Monstrum Nox is incredibly dedicated! Not only was the original port well made, but the devs also went through the trouble of adding local co-op as nifty little bonus!
Usually that would be the end of the story given that we're nearing a year since launch, but the Ys IX: Monstrum Nox team has now returned yet again with a brand new update! This time around there's no new content to toy around with, but rather some new optimizations and technical enhancements that should help improve performance by up to 25% for those with weaker CPUs.
The one catch is that this optimization requires Direct3D feature level 11_1 to properly work. As such, if you're using some very old hardware that doesn't support this, you sadly won't benefit from the new changes, though you'll still be able to continue playing Ys IX: Monstrum Nox without any trouble.
While the team has not encountered any issues during internal testing, it's also worth mentioning that the previous versions of Ys IX: Monstrum Nox are available through the beta tab on Steam. So if you run into any problems, just roll back to any of the previous “version_*” branches and you'll be good to go.
You can learn a little bit more about the new improvements, as well as Ys IX: Monstrum Nox in general, over at Steam. Enjoy!