I'm really sorry.. when I first looked at this, I did so via a code review, and it appeared that opening the Metrics Window would leave a refresh service connected even after the window was closed, which could cause the performance issue to exist even after the window was closed. When I actually tried to fix the problem, I realised I hadn't analysed it correctly, and that the service seemed to be operating correctly (i.e. the performance issue disappears when it is closed). I thus find myself in the rather unforgivable situation of having admitted to a problem that I can no longer find or fix.
The performance issue in general though (slow UI while the metrics view is open) definitely appears in every environment where a large solution is being used. A fix is planned for this in the V2 release. I'd prefer to have it in a minor release, but unfortunately it involves replacement of the entire tree component, which carries significant risk and may destabilise the product for the many people that now rely on it.