First thing's first - what exactly is the problem with the game?
Technically, it's a rush porting job, and we're not talking about being unoptimized. It's actually a very light game that will run well even on a low/mid-range rig (to give you an idea on what it takes to run the game: I have an old Core 2 Duo e8400 + nVidia GT 440 that runs it well on 1080p, with settings on what's basically medium.)
The problem with Injustice: Gods Among Us Ultimate Edition's PC Port, is that whoever is doing the porting didn't bother ensuring that the engine is frame independent. This means unlike majority of PC games these days, when the PC running it struggles to maintain the speed, it doesn't drop frames. Instead, it continues to pump out all 60 frames of the scene, resulting in slow motion instead of frameskips.
Unlike Naruto Shippuden: Ultimate Ninja Storm 3 Full Burst, which was also frame-dependent but had the excuse that the engine was coded to be frame dependent from the start and would require massive rewrites of the code, there's simply no excuse for Injustice Gods Among Us because it's using Unreal Engine 3, which was originally developed on the PC and supports frameskipping.
So yeah, Injustice Gods Among Us: Ultimate Edition suffers from slow motion gameplay even on some high end rigs because it can't skip frames like a PC game should do due to the lack of standardized hardware configuration. What can you do about it?