julia> using LinearAlgebra

julia> A1 = UpperTriangular([1 2; 3 4])
2×2 UpperTriangular{Int64, Matrix{Int64}}:
 1  2
 ⋅  4

julia> vals, vecs = eigen(A1);

julia> @time vecs * diagm(0 => vals) / vecs ≈ A1;
  1.858678 seconds (4.22 M allocations: 214.301 MiB, 8.50% gc time, 99.97% compilation time)

julia> @time vecs * Diagonal(vals) ≈ A1 * vecs;
  0.645123 seconds (1.25 M allocations: 62.933 MiB, 17.30% gc time, 99.97% compilation time)