Why is BLAS dot product so much faster than Julia loop?

General Usage Performance
15

cut and paste worked here . . .

const get_num_threads = function() # anonymous so it will be serialized when called
    blas = LinearAlgebra.BLAS.vendor()
    # Wrap in a try to catch unsupported blas versions
    try
        if blas == :openblas
            return ccall((:openblas_get_num_threads, Base.libblas_name), Cint, ())
        elseif blas == :openblas64
            return ccall((:openblas_get_num_threads64_, Base.libblas_name), Cint, ())
        elseif blas == :mkl
            return ccall((:MKL_Get_Max_Num_Threads, Base.libblas_name), Cint, ())
        end

        # OSX BLAS looks at an environment variable
        if Sys.isapple()
            return tryparse(Cint, get(ENV, "VECLIB_MAXIMUM_THREADS", "1"))
        end
    catch
    end

    return nothing
end

julϊ̇a> using LinearAlgebra

julϊ̇a> get_num_threads()
4
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