Given that the critics are mostly praising Deus Ex: Mankind Divided, one would think that everything is currently perfect, but unfortunately that is not the case. As you'll notice the moment you visit Deus Ex: Mankind Divided's Steam page, there are numerous reports of poor performance, constant crashing, and a whole host of other such annoying bugs.
Thankfully, the developers didn't just wait for the storm to pass by as they have just recently pushed out the very first bug fixing patch aimed at some of the most "critical issues", i.e. the constant crashing. Here are the brief, but hopefully significant patch notes:
• Fixed crashes caused by third-party programs interfering with the game.
• Fixed crash that occurred after viewing the intro videos.
• Fixed issue that could cause a crash at the end of the Prague intro scene.
• Fixed mouse invert setting not being displayed correctly in the options menu.
On the other hand, if you're suffering from poor performance while playing on Very High or Ultra settings, the developers have advised you first switch to the High Preset with MSAA turned off, and then tweak the options from there in order to optimize your experience. The key point here is that the MSAA option is very demanding on the hardware, so its possible you're suffering from frame rate drops due it being set too high. While this might not sort out all of your issues, its well worth giving a try.
As for Deus Ex: Mankind Divided, I would recommend you hold off purchasing it for the time being, at least until all of these issues get ironed out. Rise of the Tomb Raider underwent similar troubles, and after a few weeks of patching it was far better than anyone could've wished for, so hopefully the same will apply to Mankind Divided as well.