The glossiness parameter is similar to the one for reflections. It’s use is to blur the refractions. This is one of the most time consuming settings, render times will go really high if you use high subdivs...
Try a value of 0.8 with 8 subdivs. You’ll see that the
blurry refractions are very noisy due to the low subdivs. But you get the idea.
Turn the glossiness back to 1.0