Optimal way of using simple map() is by avoiding to use lambda function. So for example
dbl(x)=x*2
arr=rand(1000)
non-optimal method is significantly more expensive.
julia> @time map(dbl, arr);
0.000090 seconds (7 allocations: 8.125 KB)
and by avoiding the use of lambda you get
julia> @time map(x->dbl(x), arr);
0.040455 seconds (13.77 k allocations: 632.087 KB)