"what parts of flash are not open?"<p>The source code of the Adobe Flash plugin, if nothing else.<p>Incidentally, that does bring to mind one possible Hail Mary that could work: Open source the Flash player(s). Whether the open source community could fix performance issues on Linux and OSX depends on a lot of things, but they could certainly fix some things that would make it less aggravating.<p>(Since it's probably impossible to open source as-is due to licensing agreements it still might not work; depends on how much of the plugin is still left after you've removed those bits.)