Skyrim was coded in x87 code.
It does not have SSE optimization which has been standard for both AMD and Intel for nearly a decade now. Never-mind the new advanced vector extensions now available. They should have optimized for SSE 4 and AVX... not x87.
This is just a total blunder on Bethesda's part and absolute embarrassment. Unbelievable, I just can't believe they did this. Confounds me.
This is probably among the top reasons why Skyrim only runs on two threads.
Some comparison pics with FPS display:
1. Solitude Before
2. Markarth Before