Hi Guys,
This is caused by the bucket rendering we introduced in v7. In some cases Corona waits until the last bucket finishes rendering before commencing with the next pass.
We are looking at ways to address this issue in v8. However for the time being I can provide a string option so you can temporarily disable the bucket rendering.
You can put the following into the string options rollout (you'll need to enable the devel settings for this Render Setup -> System -> System Settings -> Enable devel/debug mode) int progressive.boxScramblingPassLimit = 100
The number should be higher than your pass limit, as this is the amount of passes where Corona will start to use buckets.
Let me know how this works for you.
Cheers,
Rowan