[Steeeve]

I have used Flask at significant scale for REST api requests.

I haven't done web sockets with it - what work I've done with sockets has been in Node.

For building REST apis, it doesn't get easier, IMO. It's very straightforward, it scales well, and it's simplicity makes troubleshooting a reasonable task.

It's appropriateness for slow requests may be questionable, but before spending too much time on a more robust solution it's worth looking into why requests are slow in the first place. Cacheing, message queues, etc. are easy solutions to implement. Data store optimization is generally a quick and easy win that should be done regardless. When it gets to the point where python is the limiting factor it's easy to replace because the client facing front end is generally a proxy.