Hm ... what I've seen often is that "waiting for GPU" sometimes takes up a very long time, and/or that the grey bars inside Scout start to show a sawtooth pattern. But I can't remember having seen that "clear" takes so long.
Two things: What's your stage color? That's what Starling uses to clear the context. Make sure it's perfect black (0x0), that makes a difference on some hardware.
Next, you could try if turning off VSYNC makes any difference.
Add the following code to your game, ideally right at the beginning. That "stage" is the Flash stage!
stage.addEventListener(VsyncStateChangeAvailabilityEvent.VSYNC_STATE_CHANGE_AVAILABILITY,
function (e:VsyncStateChangeAvailabilityEvent):void
{
trace("vsync change available: " + e.available);
if (e.available) stage.vsyncEnabled = false;
trace("vsync is now " + (stage.vsyncEnabled ? "on" : "off"));
});
That requires a recent AIR version.