How to speed up adding new frames?

I found a (free!) solution for you.

I opened your PDF in PhotoLine, cleaned out the clutter and backgrounds, and exported one of the meshes to an SVG (you can do the same in Illustrator, of course). Then I imported the SVG as GreasePencil into Blender, and it created a GreasePencil drawing. (Btw, it is also possible to import your drawings as true 3d splines and render them as 3d threads = takes a bit more work, but delivers interesting results).

GreasePencil mode is the 2d animation mode in Blender. It opened your drawings with all colours intact, ready to animate and cut apart in sections.

On my machine creating a new frame takes a split second. Animation is not real-time: the vectors are too complicated for that, but it is semi-realtime. An animation frame is  rendered within a second.

The mesh can be transformed in both 2D and 3D, and animated per frame. Each thread is selectable and may be transformed and animated individually.

All in all, works well, and without hiccups. And best of all: free! And FAR more powerful than Animate. It is easy to zoom in and out of the scene (all with real-time feedback in the viewport while animating)

The only drawback for you may be that you will have to learn a different animation program, so you will have to invest some time.

Fortunately, good tutorials about 2D animation in Blender are easily found:

https://www.youtube.com/watch?v=7qZWW0k41WA

https://www.youtube.com/watch?v=7t7BcmAM_2I

You may also have to do a Blender Basics tutorial. Search online: there are many.

Here is a quick recording of what the feedback and responsiveness looks like in Blender (allow the GIF animation to load once and play a second time for original playback speed at 24fps - or download the GIF file):

First, you'd better be working in an AS3 document for export as video, because you are absolutely NOT going to get realtime playback of that nightmare out of Javascript in a web browser.

Second, consider the possibility that you have selected the wrong tool for the job. Animate is designed around animating relatively simple vector shapes. You appear to have no intention of animating any of what you've drawn at the vector level, and are instead just painting on more and more vectors every frame to create a flipbook type animation. This is a task that would be better suited to Photoshop or Illustrator.

Third, if you're absolutely dead-set on sticking with Animate, the best you could do is divide your stage content into layers. That way you could stop adding keyframes for content that isn't going to change anymore, and could hide those layers for faster drawing performance. Every time you add a keyframe, Animate has to make a copy of everything that's on the stage, and it looks like you have tens of thousands of vectors on there. No wonder it's taking so long.

Finally, at the point where it started taking more than a minute to add new keyframes, that really should have set off the "I'm probably doing this wrong / there has got to be a better way to do this" warning in your head. If you don't have this warning system, work on developing one so you don't end up in absurd situations like this again.