For reasons that are not entirely clear, but probably due to performance problems on low end machines, Windows 95 tends to bundle up the messages about I/O completion and doesn't immediately wake up the application to service them. However, it does wake the application for user input, presumably to keep it feeling responsive, and when the application is awake it will handle any pending I/O messages too. Thus wiggling the mouse causes the application to process I/O messages faster, and install quicker. The effect was quite pronounced; large applications that could take an hour to install could be reduced to 15 minutes with suitable mouse input.
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