elipse draw decrease performance when radius increase

I've re-implement circle-worm demo from http://www.chromeexperiments.com/detail/circle-worm/?f= with QPaint.
Unfortunately, performance decrease when radius increase.
If you fix radius to small one, animation is good.
If you fix radius to big one, animation is slow.
If i use rect, animation is good.

If you test circle-worm demo on Browserdemo and in chrome, performance is really different. It's script compare, but whith QPaint version, you can see the same thinks.