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.