It takes 10 seconds for the boot animation to start on my setup; with or without a custom animation inserted.
Just be sure to reinstall WiiFlow Lite to Priiloader every time you make any adjustments to WiiFlow Lite's meta.xml file. The changes made will instantly reflect when booted from HBC, but the last saved configuration is used in Priiloader, regardless of the current state of the meta.xml.
The largest image size I have been able to achieve so far is 200x100, with up to 10 images used for frames of animation.
In my tests so far, anything sized smaller than 200x100 will oddly blank out the last frame in the animation sequence loop. It may also be the fact I've been using less than 10 frames in those, but 200x100 animations don't have this issue with less frames.
Seems like it's easy enough to just use a 200x100 canvas and put your smaller image in the middle, surrounded by an alpha channel transparency on the sides to compensate as a workaround for this issue.
I'm curious if it's possible to expand the maximum image size accepted. This could potentially be used for static splash screens similar to RetroPie. Larger animation frames would be interesting to try out as well.