I just read about an interesting claim from AMD's antitrust lawsuit against Intel at Slashdot. Besides dozens of other accusations they believe that Intel's code compiler makes code run slower when it detects that the processor is an AMD:
"To achieve this, Intel designed the compiler to compile code along several alternate code paths. ... By design, the code paths were not created equally.
If the program detects a "Genuine Intel" microprocessor, it executes a fully optimized code path and operates with the maximum efficiency.
However, if the program detects an "Authentic AMD" microprocessor, it executes a different code path that will degrade the program's performance or cause it to crash.""