[jmaker]

Briefly: The cost of spawning schedulable entities, memory and the time to execution. Virtual threads, i.e., fibers, entertain lightweight stacks. You can spawn as many as you like immediately. Your runtime system won’t go out of memory as easily. In addition, the spawning happens much faster in user space. You’re not creating kernel threads, which is a limited and not cheap resource, whence the pooling you’re comparing it to. With virtual threads you can do thread per request explicitly. It makes most sense for IO-bound tasks.