That is the problem of Node.JS, not a problem of React. They can run multiple instanceof of Node.JS. And if it takes 500ms to render on a server with powerful CPU, it will take even more on the client, futhermore every page update can take that much time too.

I've always found that nearly all of the time spent when requesting a page of SSR'd content (whether that be a full page or a rendered component returned to an AJAX request) was spent in the processing before the render happens. Given that, the added complexity of CSR has never been worth it for me.

This is situational, I think. My work mostly involves applications that do significant back-end processing for most requests, and my SSR is always done using a framework that has pre-compiled code doing the rendering rather than an interpreted language. (Perl and C#.) This combination adds a lot of pre-render computing and optimized rendering, which adds up to SSR being a good choice.

I'm not sure what that says about when CSR would be a good choice. If your requests don't do much back-end processing, but still have a long (500ms?) response time, that seems like you're doing something wrong rather than an opportunity to use CSR. Maybe you've chosen a poorly-performing rendering framework. Maybe you're trying to render too-large a page (which would be even more of a problem client-side.)

While the rendering time is minimal, the rendering code still blocks to wait for the underlying processing code, making the throughput low.

That doesn't make sense, unless you're talking about a specific framework that has bad performance characteristics.

Let's say you've got a request that does SSR and takes 500ms, with 450ms of that time spent processing the request and 50ms spent rendering the response. If you switch to CSR, you still have to wait 450ms to process the request, and you've got to serialize the response data (eg: render it to a more concise format than html) which is going to take some of that 50ms you're trying to save. So, where is the blocking you're talking about? How does CSR make it go away?

What you wrote sounds like you're describing a singleton that handles all rendering for all requests, and can only handle one request at a time. If that's the case, your framework is a toy and you need to ditch it for something that can handle multiple concurrent requests independently of each other.

Yes, a single process will experience the issues you mention.

You can also consider running it on a Kubernetes cluster with auto-scaling turned on.