Thanks.
The marching ants idea was Ezra's.
He thought it couldn't be done though

It's obvious which one you're working with, but you can see the content and it doesn't cover other stuff impact image quality like eg a dropshadow would.
I was going to make the animation work in all "modes" but it's quite processor intensive and I want to keep the hardware threshold low. ( Within reason, functionality and time expended developing are also important ).
If the animation is running whilst you're drawing there's a noticeable gap between the pen and the ink appearing following it.
Ink offers smoothing which is huge for anyone isn't some sort of graphics ninja.
Presumably that smoothing is also quite processor intensive.
Rather than spend weeks optimising things nobody cares about, the simple fix is to just do the animation whilst in edit mode.
If you just drew a lake or woods then you know which one you just drew.
The effect is also a bit distracting so I think this is a plus all round.