The QA on this game was clearly not strong.... Anyway, these fixes solved 100% of my performance issues. There are different registry fixes floating around but they did not fully fix the problem in my case.
Taken from Steam forums and Guru3d forums.
In regedit open:
HKEY_CURRENT_USER\Software\Eidos\Deus Ex: HRDC and locate AllowJobStealing - set it to 0
For NVIDIA owners - in HKEY_CURRENT_USER\Software\Eidos\Deus Ex: HRDC\Graphics locate AtiForceFetch4 - set it to 0
(For Nvidia SLI users only)
The game flat out does not possess an SLI profile (at the time of posting this), but you can use the one for the original Deus Ex: Human Revolution without anything catching fire too dramatically. Just find the original Deus Ex SLI profile in whatever SLI profile manager you use and add the Directors Cut executable to it.