[mrcsharp]

Well with SSE the server and client are both holding a HTTP connection open for over a relatively long period of time. If the server is written with a language that supports async paradigms, then a http request that needs async IO will use about the same amount of resource anyways. And when the response if finished, that connection is closed and resources are freed. Whereas SSE will keep them for much longer.

[nitely]

Yes, and the client may do multiple requests, and if all take long to be processed you may end up with a lot of open connections at the same time (at least on http1), so there is a point to fast HTTP requests+SSE, instead of slow requests (and no SSE). Granted, if the server is HTTP2 the requests can share the same connection, but then it'd be similar to just using WS for this usage. Also, this allows to queue the work, and processed it either sequentially or concurrently.

By async I meant a process that may take longer than you are willing to do within the request/response cycle, not necessarily async IO.

[mrcsharp]

You make a good point about queuing up the work. You do get more control over resource management in this case.