If I launch an html embedded swf file on one server, my client's idle CPU for the browser/plugin-container hovers around 18-20%. If I take the same swf file and move it to another server, and use the same browser, the client's idle CPU is 0-1%.
The plugin is idle when I'm looking at Windows Task Manager--I'm not clicking anything, there are no server calls. It doesn't matter what browser I use, and I've tried from two different client machines and have seen the same result.
What could make the client CPU differ?
Did you run the test with a simple "helloworld" SWF?