<instruction sets>
While I'm reasonably certain about the truth of the above, I probably butchered the terminology somewhere... But I digress..
You're pretty much completely wrong. The core instruction set, x86, is pretty much completely unchanged since the 80386 was released in 1985, and all PC CPUs, Intel or AMD, use identical implementations. Now, both Intel and AMD have introduced many extensions over the years, such as SSE/SSE2/SS3/etc, x86-64 (64-bit support), and while not every CPU model implements every single one of them, pretty much all of them are licensed to both companies, and there's not any sort of 'emulation' going on that makes AMD implementations of Intel extensions (or vice-versa) necessarily worse (they may even be better). Furthermore, the differences in implementations shouldn't affect DF performance at all -aside from 64-bit support, they're pretty much all focused on intensive floating point calculations, of which DF is doing few, if any.
There certainly will be differences in the performance of DF on different CPU architectures, but that will be decided not by what machine Toady uses to compile it, but by the particular strengths and weaknesses of that architecture and how they align with with what DF actually does. The only way to determine that would be scientifically measuring it, benchmarking the performance accurately on the different CPUs.