`cat` allocates too much memory

[DNF]

@ArthurW

julia> @time reduce((x,y)->cat(x, y, dims=3), A);
  6.262270 seconds (526.79 k allocations: 18.537 GiB, 3.85% gc time)

So why is cat so much slower than vcat:

julia> @time reduce((x,y)->cat(x, y;dims=3), A);
 12.606505 seconds (526.91 k allocations: 18.537 GiB, 16.69% gc time)

julia> @time reduce(vcat, A);
  0.038854 seconds (2 allocations: 59.815 MiB)

?