The dispmanx backend for SDL1 is unrelated to the RetroArch dispmanx driver (apart from the fact they both render with dispmanx api). It can actually decrease performance due to the way it forces double buffering. (There is a newer dispmanx backend for SDL available we don’t have yet that should have better performance).
Using the dispmanx backend for SDL1 can help some stuff which is unstable when using framebuffer directly, and can stop tearing as it forces vsync. It will allow rendering to a smaller resolution and then scale automatically. Although the framebuffer has hardware scaling also, it requires modes to be set up first and can have problems with some resolutions.