A Last August I was looking at:
and performance was truly abysmal. Reading that thread, it took two minutes to fill zones. After some detective work I found:
Examining the F.Cu shows “Kevin Santo Cappuccio”. If that object is removed, zone filling on the freshly loaded Rev. 3 board is back to 5 seconds.
Back then I was thinking about creating a bug report for this, but it got buried under all the noise in my head.
The text string that caused the problem was a high vector count polygon. It is quite possible that the eagle to KiCad conversion generates similar things.