Jasmine environments have a default updateInterval value of 250 that determines how often, in milliseconds, execution of the next spec will be deferred so that the screen can be updated.

    var now = new Date().getTime();
    if (self.env.updateInterval && now - self.env.lastUpdate > self.env.updateInterval) {
      self.env.lastUpdate = now;
      self.env.setTimeout(function() {
        self.next_();
      }, 0);
    } else {
      if (jasmine.Queue.LOOP_DONT_RECURSE && completedSynchronously) {
        goAgain = true;
      } else {
        self.next_();
      }
    }

Both Chrome and Firefox now require a minimum value of one second for setTimeout in a background tab. This basically means that for every 250ms of work that we do, we end up sleeping for 1000ms.

This gist shows one way to tell Jasmine to not even bother trying to update the screen when running in the background.

var foregroundScreenRefreshRate = 1500;
var backgroundScreenRefreshRate = 9000;

jasmine.getEnv().updateInterval = foregroundScreenRefreshRate;

$(window).focus(function() {
    jasmine.getEnv().updateInterval = foregroundScreenRefreshRate;
});

$(window).blur(function() {
    jasmine.getEnv().updateInterval = backgroundScreenRefreshRate;
});

(Please refer to the gist for the most up-to-date code.)

This code makes Jasmine run at full-speed in a background tab in Chrome, but continue to be updated about once every 2.5 seconds when in a foreground tab. However, using this as-is in Firefox will result in a warning about an unresponsive script, if the tab is inactive. Luckily, you can continue to run Firefox in the foreground fine with this script (good for CI perhaps), or you can just override the dom.max_script_run_time variable to never get that warning, or you can set updateInterval to something less than the default 10 second max script run time.