"When problems resist parallelization or have no appreciable latency to hide, the third way that concurrent execution can improve performance is to increase the throughput of the system. Instead of using parallel logic to make a single operation faster, one can employ multiple concurrent executions of sequential logic to accommodate more simultaneous work. Importantly, a system using concurrency to increase throughput need not consist exclusively (or even largely) of multithreaded code. Rather, those components of the system that share no state can be left entirely sequential, with the system executing multiple instances of these components concurrently. The sharing in the system can then be offloaded to components explicitly designed around parallel execution on shared state, which can ideally be reduced to those elements already known to operate well in concurrent environments: the database and/or the operating system."
Even if you can't use the database or os and still want to get "multiple concurrent executions of sequential logic" then just use a message queue. That's what Go provides, what blocking queues in java provide, what most actor based systems provide (e.g. Akka, Dart), what unix pipes provide and what all kinds of inter process messaging systems provide. Most of your code can as impure, sequential and lock free as it always was. You only have to make your messages immutable.