You mentioned in your question the option spring.virtual.threads.enabled=true. Just to clarify, you mean spring.threads.virtual.enabled=true right?

Also, if you would like to return the response immediately, may I propose to change your code from:

@GetMapping
    public ResponseEntity<String> hello() {
        log.info("Received request in controller. Handled by thread: {}. Is Virtual Thread?: {}",
                Thread.currentThread().getName(), Thread.currentThread().isVirtual());
        helloService.hello();
        return ResponseEntity.ok("Hello, World!");
    }

to something like:

@GetMapping
    public ResponseEntity<String> hello() {
        log.info("Received request in controller. Handled by thread: {}. Is Virtual Thread?: {}",
                Thread.currentThread().getName(), Thread.currentThread().isVirtual());
        CompletableFuture.runAsync(() -> helloService.hello());
        return ResponseEntity.ok("Hello, World!");
    }

I tested on my local and this works.