I seem to be getting beach balls (and the Animate app hangs) about 50%+ of the time when I attempt to test (⌘+↵ or Control > Test) a HTML Canvas movie on macOS (El Capitan v10.11.6). It'll often recover 30 seconds or so later (usually after switching between other applications) but it's annoying enough that it's a big productivity killer when using Animate (actually it's enough to avoid using Animate altogether).
If you have publishing set to create spritesheets, and you have lots of bitmaps, it can take a while to create the spritesheets. You could turn that off for general testing, and then back on for the final publish.
I had wondering if that might have been the problem so I ran a few tests and realised that it happens even when it's only a single, simple shape (or image) on a single frame.