What? A hundred on screen objects is not a lot, and 20fps is a slideshow. Pure Java games regularly do 10x that (Minecraft says hello as a simple example, and it's considered badly optimized)
A "proper" C/C++ engine you'd expect 100x (thousand objects at hundreds of FPS, or things like Factorio which are 10s of thousands of objects at 60 fps).
The anecdote performance is what I'd expect from a joke language like Rockstar. I'm sure the poor performance is not Closure's fault necessarily, and it probably can fly in the right circumstances (or maybe even in a lot of circumstances). Just the anecdote's description makes Closure sound horrifically slow.