The canvas plugin is causing the cpu usage when webgl is on. Basically it has to copy the canvas to a texture every tick. If you disable webgl it's fast or alternately you could use the paster object instead.
Nice examples. I didn't think of using the circle for the rainbow, but it looks effective. The only issue you'll run into by fading the canvas like that is the images never fade away completely do to rounding, unfortunately there's nothing that could be changed to make it better. It's more noticeable on certain monitors or if you look at the screen from an angle. Still it's a good effect that I'll use again.
If you're interested in existing solutions you can look here for one using sprites:
It can be tuned to not use as many objects if performance becomes an issue.
For trails with textures the paster object would need to be used.
Paster's draw textured quad action handles this well, notion special needed to be done for repeating textures.
https://www.dropbox.com/s/umtu776abnzc0 ... .capx?dl=1