Whenever a project file contains multiple drawings with paint brush tool, it increases the file size and hence you may face some performance issues with Animate.
To avoid Animate CC performance issues while using paint brush tool, Animate team recommends the following best practices:
Reduce the number of paint brush strokes used in symbols. You can avoid performance issues when you use object drawing mode of paint brush tool.
When a drawing tool is in Object Drawing mode, the shapes you create with it are self-contained. Animate creates each shape as a separate object that you can individually manipulate.
For more information, see Objects drawing mode.
To convert lines to fills, select a line or multiple lines and select Modify > Shape > Convert Lines To Fills. Selected lines are converted to filled shapes, which allows you to fill lines with gradients or to erase a portion of a line. Converting lines to fills can increase file size, but it can also speed up drawing for some animations.
Also, you can select the tool and choose Draw as fill in the Property inspector panel.