The following text is ripped directly from the Source 2 group on Steam
Source 2 includes technology for rendering improvements, performance optimizations, higher fidelity content, and richer, more dynamic games.
Source 2 was built to support a wide range of hardware. Source 2 has been built to be capable of driving modern machines to their limits. It can use all of your CPU cores, your 64-bit OS and memory, and includes support for recent and upcoming graphics standards like Direct3D 11 and Vulkan as well as virtual reality if the game demands it.
Multithreading and Streamings
Source 2 will use any available CPU cores to provide a smoother experience, removing many hitches during gameplay. Also, much of the game content can be streamed in the background while the game stays responsive. Animation, voice chat, music and more can continue while the game is loading.
Source 2 uses a new audio system that processes all audio in full quality through the entire pipeline. Voice chat makes use of this improvement, and is now noticeably higher quality and has less latency than before. Audio and voice processing also use multi-threading to run in the background and avoid dropouts. The Source 2 audio system also makes use of your processor's SIMD instruction sets to improve performance. Additionally, it offers new audio scripting capabilities for audio designers which lets them customize the mix and effects pipeline without changing any code.
The renderer in Source 2 contains a number of new features and improvements, giving creators more design freedom. Maps in Source 2 can be authored using powerful geometry editing tools and may contain complex meshes which are free of convexity constraints. The engine is capable of delivering many more batches to the GPU, even when using older versions of the OpenGL and Direct3D graphics APIs. This makes it easier to create more detailed worlds and more advanced characters. Additionally, a unified lighting system ensures all game assets have access to the same rendering feature set, ensuring visual continuity.
Source 2 also includes new versions of our physics and cloth simulation engines which implement higher quality, more stable simulations.
Source 2 includes native support for OpenGL without needing a wrapper, improving performance, memory usage and loading times on other platforms.