Here is a concurrent connections benchmark showing Elixir and Clojure (both based on immutable data structures) outperforming every garbage-collected mutable-arrays language in the benchmark. (Only C++ outperformed the immutable-collections languages, which is presumably less about data structure choice and more that C++ never has to pause for garbage collection.)
Assuming mutability of sequential collections were somehow a significant bottleneck in a server’s ability to do low-latency high throughput processing of many concurrent connections, the focus of any concern should be whether Java’s slow and clunky mutable arrays can hope to compete with the obvious superiority of Elixir and Clojure’s immutable collections. 
Just to be super clear about it: the real point here is that mutable arrays don’t make a difference in this domain. It’s all about how the servers and VMs are architected.