Code that loads on demand starts at once, images decode beside their downloads, writes run compiled

A page that loads part of its code only when it needs it asks for that code with a dynamic import. The browser used to fetch each such request one after another on the page's main thread when its turn came, so three imports on a slow connection meant three waits in a row with the page frozen. Now each import starts fetching on a background thread the moment it is called, imports asked for together download together, and the page keeps running meanwhile.

Every image was decoded on the thread that loads the page, one after another once its bytes had arrived. Each image now decodes on the background thread that downloaded it, as soon as it lands, so a page with many large pictures is ready sooner and the loading thread stays free for the page's own work.

When a script measured a page while something on it was animating, every animating element had all its style rules recomputed, because an animation's previous frame left no record of the styles underneath it. Each element now keeps its styles from before the animation, so a new frame starts from them directly, and an animation that ended in between is dropped exactly. Style maps are also shared until something changes them, instead of copied whole.

Any function that wrote to an object, as in o.x = v or o[key] = v, was refused by the compiled tiers and ran in the interpreter, however often it was called. Writes now run compiled, through the very same rules the interpreter applies (setters, frozen and sealed objects, proxies, arrays), and every one of those rules is checked against the reference engine in every tier. Small helper functions such as type checks now run inside the compiled code that calls them, with a guard that falls back to a real call if the helper is ever replaced.

When a script reads a size right after changing the page, the browser lays the page out before answering. The performance profiler used to charge that layout to whichever function happened to run next, so a one-line type check could look like the slowest function on a news site. Forced layout and style now appear as their own entries, directly under the function that forced them, and the rest of the profile is unchanged.