There have been graphical improvements added, particularly PBR lighting and cloth textures. There will be optimizations made later in development, but I don't know how much that is expected to improve performance, or if there are any performance implications for the new camera system being implemented, or the ongoing updates to animations, textures and effects.

Check again after release, but in D:OS EE, playing on high quality (1920x1200), I get 59-60 fps in-game, outside of combat (i5 3570K, Radeon HD7870). With D:OS 2, in the current release (3.0.15.252) I get 45-55 fps, sometimes getting up to 60, and in areas with a lot of NPCs around and patrolling guards, etc, it can drop to 30-40. Lowering the quality to medium adds 10-15 fps to those ranges (with little difference in visual quality, that I can see).