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When we upgraded from 6.x-2.9 to 6.x-3.x-dev I noticed a severe drop in page performance. We used 1 nested fieldgroup and page generation time shot from 400-500 ms to 900-1100 ms.
I'm not sure if this is a known issue but I assume you'd like the feedback.
We've downgraded again to 6.x-2.9 and performance is restored.