Improving runtime

[gdalle]

At first glance, it seems your code performs a lot of unneeded allocations. For Julia to be fast, you want to avoid allocating new memory when you can reuse it instead.
See the performance tips for more details, especially the sections about

• pre-allocating outputs
• fusing vectorized operations
• using views for slices

In many cases it will be as easy as replacing

for i in 1:n
    x = y - z
end

with

x =  # init
for i in 1:n
    x .= y .- z
end

whenever you deal with arrays