Very high memory usage from gnome-shell

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When I boot into F22 gnome-shell, it starts at 60 MB. Then, as I interact with my programs, it goes around 200 MB (the average gnome-shell usage on my end). However, once in a blue moon, the entire desktop environment will start lagging and stuttering, and gnome-shell will be the culprit behind that, at more than 2 GB of memory usage.

Why does that happen? I check the problem out on various online forums, and it seems like the Nvidia drivers are what often cause the problem. However, I am using Radeon graphics. I therefore fail to see how the Nvidia drivers would affect me.

Thanks.