Raw speed these days means concurrent processing, so those two are more and more often the same case. The whole "rewrite it in Rust" trend is a very clear example of the benefits of easier correctness of concurrent programming - Rust programs end up being faster than other alternatives even though on paper C has better "raw speed" (e.g. no bounds checking).
1. Raw speed on modern CPUs means taking advantage of data locality more than anything else. Even concurrency. Cache misses will cost you a few hundred cycles, far too much to make up for with concurrency in most cases.
2. Of course given a sufficiently large array, iterating over it with 16 processors is faster than with 1. Arrays still dominate other data structures for raw performance here.
3. Concurrency doesn’t just mean multi threading. SIMD instructions can perform simultaneous operations on multiple operands in your array. Can’t do this with a linked list.
Yes you can write a very fast SIMD loop over densely packed data. But if that data is mutable and you need to acquire a lock before you work with it, it's very easy to lose all the performance you gained. Immutability can reduce coordination costs and improve effective parallelism.
For a similar reason immutability also helps you write code with fewer data races.
A single threaded SIMD loop over densely packed data, will outperform the same transformation on a linked list running on 50 threads (this obviously an over generalization and there are transformations and data layouts where this doesn’t hold, but it’s very common. You could also construct cache line aware hybrid data structures, but there are trade-offs).
The only reason you’d need to deal with increasing parallelism (beyond SIMD) is if you wanted it even faster than that.
I’m not saying immutable data isn’t a good idea in many cases (my primary day job language these days is Elixir). What I am saying is that if you are “optimizing for raw speed” immutable data structures are almost never the right choice.
That doesn’t mean immutable data structures can’t be fast enough to be the best choice in many situations.
