Maybe plug-in execution ought to stop when the relevant page is in the background. At least on battery-powered devices. Some Flash content tends to run in the background, doing who knows what, probably something related to advertising, tracking, or general obnoxiousness. On phones, "Run in Background" is usually a privilege that's discouraged.
Many people expect to be able to listen to say, music on youtube in one tab, while browsing the internet in another. So it's a bit of a hard problem (not to mention that a plugin just keeps running, and you have no real control over it). However, all the major browsers try to throttle javascript in background tabs. Usually by making setInterval return only once a second.
