Here is some info about the CPU bottleneck in FSX.
Originally Posted by Phil Taylor
One source of bottlenecks are the per-frame number of D3D API calls. Which can cause a block in the driver on the CPU-side.
Part of the SP1 work is to reduce these calls.
The DX10 release will give additional reductions due to the better instancing, and due to the reduced per-API call overhead.
Senior PM, Graphics and Terrain
Aces Studio, MGS