Note that when you use square-bracketed indexing as the left hand side of a = assignment, it’s not doing getindex (and thus isn’t materializing the array). In the case of A[I, J] = X, it’s doing setindex!(A, X, I, J), and in the case of A[I, J] .= X it’s doing (essentially) broadcast!(view(A, I, J), identity, X). It’s the view with CartesianIndices that’s slow.