'Optimised', huh! Thats just weasel words for 'I need a better graphics card'
M2TW engine is just doing much more than RTW engine.

RTW engine is essentially a pure DirectX 7.0 renderer.
It uses Vertex Shader 2.0 if present to help with all the soldiers but the lighting etc is all DX7 standard per-vertex, no shaders.

M2TW is doing per-pixel lighting, bump, reflection & other shaders such as water & windows, all DX9 stuff that uses vastly more computation power than per-vertex fixed function as used by RTW engine.

Add to that the higher texture resolution, higher poly count per soldier, much more detailed terrain/vegetation & buildings.

That said, EB 1.1 on the RTW engine manages to cause my ATI 3870 to crash at the overclock I run everything else at, have to downclock significantly to keep stable.
This is RTW engine pretty much at max & its able to still put a lot of load on a high-end late 2007 GPU.