I don't believe the ISA dictates the implementation, so theoretically someone could implement the D extension to get 64-bit precision floating point in a more p

I don't believe the ISA dictates the implementation, so theoretically someone could implement the D extension to get 64-bit precision floating point in a more performant way than other implementations. If someone altered the ISA in order to get performance, I don't think you could accurately call it RISC-V anymore.