Hi,
We had have the same reflexion, before read your post, I've just put off the "for each" loop in the capx and make the sprite smaller (I update in Scirra Arcade too...). But I don't have any amelioration without :(
I win a very very little but almost nothing.
I think the problem is somewhere with may be my chipset and C2 ? very fast with GM on my small laptop but not with C2.
If you want, here's the GM HTML 5 Project : Download the GM HTML 5 project
You will see that is the same algo. I don't know why. I reapeat, my laptop is a "small laptop" with gma 3150 graphic card. But I think if I can get it fast with GM I should have the same with C2. That's why I think about a "compatibility issue" but witch ? ^^
Or, how you say may be GM and C2 have their differences that make it work in a different way. I really don't know.