Why const global is better than immutable struct field?

mwe-0.jl

const H  = 20
const W  = 20
const f  = zeros(9, H, W)
const rho = ones(H, W)

struct Lattice
    f  :: Array{Float64, 3}
end

const lattice = Lattice(f)

function moments1()
    lf = lattice.f
    let n, m, i
        @inbounds for m=1:W, n=1:H
                local a = 0.0
                for i=1:9 a += lf[i, n, m] end
                rho[n, m] = a
        end
    end
    return nothing
end

function moments2()
    let n, m, i
        @inbounds for m=1:W, n=1:H
                local a = 0.0
                for i=1:9 a += f[i, n, m] end
                rho[n, m] = a
        end
    end
    return nothing
end

using Einsum
function moments3()
    lf = lattice.f
    @einsum rho[n, m] = lf[i, n, m]
    return nothing
end
function moments4()
    @einsum rho[n, m] = f[i, n, m]
    return nothing
end

using BenchmarkTools
@btime moments1() # 391.292 ns (0 allocations: 0 bytes)
@btime moments2() # 387.315 ns (0 allocations: 0 bytes)
@btime moments3() # 1467.   ns (0 allocations: 0 bytes)
@btime moments4() # 387.108 ns (0 allocations: 0 bytes)

mwe-1.jl

const H  = 20
const W  = 20
const f  = zeros(H, W, 9)
const rho = ones(H, W)

struct Lattice
    f  :: Array{Float64, 3}
end

const lattice = Lattice(f)

function moments1()
    lf = lattice.f
    let n, m, i
        @inbounds for m=1:W, n=1:H
                local a = 0.0
                for i=1:9 a += lf[n, m, i] end
                rho[n, m] = a
        end
    end
    return nothing
end

function moments2()
    let n, m, i
        @inbounds for m=1:W, n=1:H
                local a = 0.0
                for i=1:9 a += f[n, m, i] end
                rho[n, m] = a
        end
    end
    return nothing
end

using Einsum
function moments3()
    lf = lattice.f
    @einsum rho[n, m] = lf[n, m, i]
    return nothing
end
function moments4()
    @einsum rho[n, m] = f[n, m, i]
    return nothing
end

using BenchmarkTools
@btime moments1() # 423.995 ns (0 allocations: 0 bytes)
@btime moments2() # 214.657 ns (0 allocations: 0 bytes)
@btime moments3() # 1471.   ns (0 allocations: 0 bytes)
@btime moments4() # 214.623 ns (0 allocations: 0 bytes)