I'm not experienced with Node.JS, but something hogging the main event loop of an asynchronous server with a long computation sounds horrible. It'll mangle your throughput, and it'll create some really strange latency spikes for unrelated requests, because the slow computation blocked the event thread from delivering the response of another request. Is this not as much of a problem practically, or would you isolate that SSR code from REST code in different application instances?

In a threaded server like java application servers or go applications, that should be a non-issue since requests are handled in mostly independent threads. It might increase the necessary compute resources some, but that's expected when moving work to the server side.