A program in Windows makes extensive use of standard libraries that belong to Microsoft Windows. These libraries are rewritten and implemented in Wine so that they do much the same. The more a program is well written and the standard libraries, the greater the chance that the program will work in Wine.
The code of a program is first examined for calling the Windows libraries. These calls are translated so that the Wine's libraries called. Then the code of the program executed at full speed. There is no emulated code, but only the calls to the libraries are translated.
Therefore, in theory. a program should work about as fast as Microsoft Windows. In practice, the program may be slower, because the translation is made. But there is a possibility of a program working faster because the libraries of Wine are simpler, and because disk access is often faster in Linux.
Because Linux uses OpenGL, Windows programs that use OpenGL often work well in Wine. Linux does not use DirectX - however, DirectX games work in Wine because the DirectX calls can be translated by Wine to OpenGL calls.