It is called downstream to indicate that the application performs its functions after all backend and database processing has completed. A firewall server is an example of a downstream application because it is runs as an application that is external to your Web server, and performs its work after a document has been served. A firewall server has a much simpler job to do than an application like WebPlexer, but in principle, they are both very similar. In fact, this is the minimum requirement to use WebPlexer. No, WebPlexer is a software application that runs on the same computer as your Web server.
Network delay is determined by the size of the pipe from the Web server to the browser.
Rotational delay of the hard disk is a function of the mechanics of the device. These delays are orders of magnitude slower than any delay contributed by CPU processing. WebPlexer is a lightweight, CPU process that makes extensive use of in-memory caching. As such, any additional delay that it might contribute is imperceptible compared to network and rotational delays. WebPlexer uses the Apache Web server for its transaction processing and HTTP communication.