I tried to reproduce the issue in Firefox, and I only got the browser lagged for 5-6 seconds when I tried to edit Group of rules with the firebug turned on. I guess this is most probably a plugin related issue.
In my case the lag was short enough to say that it is not a big deal. In your case, it seems a bit longer so I would like to ask you what other plugins you have installed and whether firebug was on while you were experiencing the issue. Still i think some other plugin must affect this issue as well, because the difference is quite big.
I am looking forward to your input so I can explore the issue in more details.
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