I don't know if it's poorly coded, or the fact that it doesn't like ATI/AMD GPU's?
I also read that RW3 wasn't going to significantly hurt performance compared two RW2, but the opposite is clearly true.
My very high-end PC (which could easily run RW2 @ 50+ FPS all day long @ 2560x1440res /8xFSAA/16x Anso.filtering) now
struggles to reach 25-30fps with HIGH settings and no FSAA or Anso. enabled

Multi-Core is indeed broken, but this isn't quite a surprise as I did read the other day (before RW3 was released) that there is going to be an update
to allow this feature to function properly. This was something the RS team posted.
Overall I'm quite happy with the visual improvements made to RW3, but the overall performance needs some tweaking and FSAA/Anso. needs to be
fixed and/or properly implemented. (Solid Crossfire / SLI support would be good too).
My System specs:
OS: Win 7 Ultimate 64bit
PSU: Corsair AX1200
MOBO: Rampage III Extreme
CPU: Core i7 990x (@ 4.35GHz)
CPU Cooler: Corsair Hydro H100
RAM: 18GB - Corsair Dominator GT (@ 1805MHz)
GPU1: AMD Radeon HD6990 (CrossFireX)
GPU2: AMD Radeon HD6970 (CrossFireX)
SSD1: OCZ Vertex 2 - 180GB
SSD2: OCZ Vertex 2 - 240GB
HDD: WD Black - 2TB