Apple used to ship a dev app called "Spin Control" that would log stack traces whenever an app failed to drain its event queue in a timely manner (i.e. beachball). One time I accidentally left this open for an entire week, went about a bunch of assorted business, and when I came back every single stack trace had to do with flash, and there were many. Either flash in a browser or flash in a browser embedded in something else (ads embedded in 404 pages for broken help pages that were never even displayed, lol). At first I thought it had to be a mistake, a filter I had forgotten about or something, so I triggered a spin in Mail.app by asking it to reindex and sure enough that showed up as the first non-Flash entry in Spin Control.
As hard as it was to believe: Flash had been responsible for every single beachball that week. Yikes.