Well, it does! In fact, the graphics performance was equivalent to my $400 windows laptop!
On Raspbian: (slow and laggy)

On Chromebook OS: video link Perfectly smooth, not slow or laggy.
What made the difference? If I understand correctly, Chromebook OS is hardware accelerated, Chromium in Raspbian is not.
Below is a screenshot of chrome://gpu in Raspbian Stretch:

Doesn't look too good.
Now what does chrome://gpu look like on chromebook OS?

Less red, more green. I think WebGL and WebGL2 are make the most difference in terms of Scratch 3.
So we know chromebook OS runs Scratch 3 better than Chromium in Raspbian. And we think that it is due to WebGL being hardware accelerated.
Here is my question: Can we figure out how chromebook OS's hardware acceleration works, and then take that knowledge and apply it to chromium-browser in Raspbian?