You would typically use something like nginx as a proxy, in conjunction with uwsgi for managing a number of workers, and then offload slow operations to a task

You would typically use something like nginx as a proxy, in conjunction with uwsgi for managing a number of workers, and then offload slow operations to a task queue via redis or something similar. Caching obviously helps if it's applicable and it's also easy to expand to multiple servers with a load balancer. Websockets are a little bit more complicated but definitely possible. As a side note, the synchronous request processing is more a consequence of Python rather than flask itself. I've personally found that I build more scalable things in Python, compared to something like node, because it lends itself to scalable architecture decisions. You can do a lot of things in node that are super convenient when you have a single server but that require major changes when you expand.