Just like with Flash, JavaScript code runs by default on the UI thread, and any expensive computation will usually affect the UI responsiveness. As you may know, at 60 fps, you have around 16ms (1000ms/60) per frame to do what you have to do (computations, rendering and other misc logic). If you exceed that budget, you will alter the frame rate and potentially make your content feel sluggish or worse, unresponsive.
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