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?
Technically, the snarky reply is to upgrade your PC so much that you'll never experience one iota of slowdown on the game, which means everything runs buttery smooth. But it doesn't have to be that expensive, as users all over the net have found the following steps to solve the problem:
First, you need to turn off shadows completely. Don't just reduce it. Turn it off. There's something about the shadows in the game that completely trashes the performance, even on high end rigs. You don't have to worry about the game looking like butt, the game still looks good even with the shadows turned off.
Turning off the shadows will fix things for majority of people suffering from the slow motion, but if you're still getting slow motion gameplay, you probably have weak hardware. In which case you should also do the following (note that this will have a more pronounced effect on visual quality, so you should decide whether removing the slow motion effect is worth the reduction of visual quality):
- turn off V-Sync in your catalyst control center or nvidia control panel and do not select v-sync in game.
- turn off Anti-Aliasing.
- if you are on a laptop make sure it is plugged in and set to high performance mode.
...and that's it. You should have smooth as butter gameplay on Injustice:Gods Among Us now, assuming that your PC is actually capable of running the game. No amount of tweaking will make the game run on a Pentium 4 + Intel GMA 950.