CPU tasks (threads/processes) are quite heavy, for short and bursty connections the async model is a bit better in terms of memory usage but takes a bit more CPU in exchange (since you're effectively doing task scheduling on top of the OS task scheduler)

Multiprocessing has a lower CPU overhead and is usually the better choice if you have less but heavier connections (ie do a lot of work on each connection) since the OS scheduler can then properly allocate resources to each process (ie, providing a minimum amount of CPU time to all processes so no connection gets stuck). Async can do that too but it doesn't have the resource allocation granularity that processes have (In go or JS I can't just allocate less resources like CPU time to a connection)