I have no particular opinion on the teaching materials, I just use the manual and the discussion fora, so I don't know. But if a third party offers teaching materials, it's not so strange if it resides on their third party website.

As for performance, I'm not really talking about 'optimization'. Your implementation may simply have used some pattern that should be avoided, such as global variables, type instabilities, abstract types in structs, or some inappropriate data structures. If it's a microbencmhark, then there are some things to keep in mind.

These are not really optimizations, but basic performance principles. I cannot know that you are unaware of them, but your statement that 'there's not a lot to optimize' make me suspect that this could be the case. The unusual thing about Julia is that it's both dynamic and compiled, so that code that would simply not compile in static languages instead ends up slow.

If I had to guess, your problem is type stability. Are you using NamedTuples to store your state and items you’re iterating over? If the keys and are not all the same and value types don’t stay the same (e.g. something initialized as zero(Int) and then accumulated into with Float64s) then performance will suffer. Another possibility is that you have a data type not is not concrete in an inner loop. For example, Array{Real} will be slower than Array{Float64} because an array of Reals has to support arrays mixing Float32 and Float64. If you had this in a function definition the likely correct thing to do is Array{<:Real}, which means the element type of the array must be a subtype of Real. Maybe even better, just drop the type annotations. They very, very rarely improve performance because Julia relies on compile time type inference.

Failed or bad type inference is almost always the cause of performance issues in Julia. Getting a feel for when the compiler can infer things or not takes practice, but it’s a lot easier than the semantics of generic programming systems IMO.

The REPL is really great for learning. If you type “Array{Int} == Array{Int, 1}” the result is false. If you type “?Array” it prints the docstring which gives some guidance on how to use one versus the other.