Blender 2.8 Lag issue on grease pencil with multiple layers. Possible temporary solution There is a good amount of lag when drawing extensively.
Adding lines isn't slowed down thankfully but erasing is 
a notable problem as well as sculpting.

Until 2.8 is optimized for performance,
there is a possible solution I don't think
would take too much time to implement.

When animating, I have a huge workflow problem when 
I start building up too many lines and layers.

Some scenes are finished and don't need to be edited further.
Therefore, they don't need to be actively rendered real-time in the viewport and take up more resources while I work more on the project.

For those key frames, that object could be converted to some sort of "smart proxy"-esque low quality version to have representation WHILE not tax the program. Maybe there could be a select an object and some keyframes and click
"covert to proxy" then all the frames are rendered and then become low quality temporay pngs then can represent that spot.

Rendering lines in the 2D animation tab in the viewport seems to be hugely slowing down work flow. I think this would save resources and be less taxing on the software
I know what your saying, I run into a tonne of lag when animating grease pencil in full scenes. My favourite way so far to get around this is to finish of the main parts of the scene that effect your character and view (e.g Camera animation and interactive objects, then save the scene then strip the scene right down and save it again using a file name that is relevant to the file (GP animation etc.) then do your grease pencil work. Once finished save and go back to you main file create an empty and then "Proxy" the GP collection into your scene and append it to your empty. Move the empty to the desired position and render. Hope this helps