"both 'don't share' and 'don't mutate' techniques tend to increase copies of data, which incurs runtime and garbage collection costs that both impact latency. You gain lower maintenance costs, but it's very hard to maintain high throughput together with low latency with those two techniques. If you need low latency, you need 'coordinate access' and, more specifically, coordinate access using lock-free techniques."
"SSD throughput is typically 2 orders of magnitude higher than HDD, but you can gain up to a further 4x speedup by making the application SSD friendly - eg using multiple concurrrent I/O threads (for small reads; large reads should keep the concurrent I/O thread count low). This differs markedly from HDDs where multiple concurrrent I/O threads tends to actually decrease throughput."