Rosetta uses software emulation for x87 floating point. That's slow, but in practice that doesn't matter much. Mac software never had a reason to use x87 FP, every Intel Mac had at least SSE3 support.

There was at least one reason...

    long double x87me(long double a, long double b) {
        return a+b;
    }

    pushq %rbp
    movq %rsp, %rbp
    fldt 32(%rbp)
    fldt 16(%rbp)
    faddp %st(1)
    popq %rbp
    retq