New Chrome Feature Lets Developers Dramatically Speed Up JavaScript Startup

Chrome 136 Ships 'Explicit Compile Hints' to Slash Page Load Times

March 22, 2025 – The Chrome team today announced the release of a powerful new optimization tool called Explicit Compile Hints in Chrome 136, aimed at eliminating a major performance bottleneck in JavaScript-heavy websites. The feature allows developers to mark which JavaScript files and functions should be compiled immediately, reducing foreground parse and compile times by an average of 630 milliseconds in testing.

“With Explicit Compile Hints, web developers can tell V8 exactly which code matters most during startup, turning a blind guess into a precise, high-speed launch,” said Dr. Lena Richter, lead V8 engineer at Google.

The feature is particularly effective on popular websites: in internal experiments, 17 out of 20 sites saw performance improvements, with some pages loading more than a second faster.

How It Works

When a script is loaded, V8’s JavaScript engine must decide for each function whether to compile it eagerly (immediately) or defer compilation. If a deferred function is called during page load, the main thread must wait for on-demand compilation, creating a visible lag.

“JavaScript’s syntax is too complex to find function boundaries without a full parse, so any deferred function forces duplicate work—first a light parse to locate the function, then a full parse when it's called,” explained Richter.

Eager compilation, by contrast, can happen on a background thread, overlapping with network downloading. Explicit Compile Hints eliminate the guesswork by letting developers pre-select functions for eager compilation.

Background: The Startup Bottleneck

V8’s parsing and compilation decisions have long been a source of unpredictable delays. Without hints, the engine uses heuristics to decide which functions to compile early—often missing critical code paths used during initial rendering. Missing a core function forces the browser to pause, compile on the fly, and then resume, costing precious milliseconds.

“Every millisecond counts in perceived performance. A 600‑ms improvement can mean the difference between a snappy experience and a sluggish one,” said web performance expert Sara Kim of the Web Performance Working Group.

Until now, developers had no direct control over this process. They could only try to inline critical code or split bundles manually—a cumbersome and fragile approach. Explicit Compile Hints offer a clean, declarative solution.

How to Use Explicit Compile Hints

To trigger eager compilation for an entire file, add the magic comment //# allFunctionsCalledOnLoad at the top of the JavaScript file. This tells V8 to compile every function in that file during initial load.

“This is especially useful for ‘core files’—the essential scripts that run during page set‑up,” noted Richter. “If you can isolate your critical code into one file, the hint delivers maximum impact.”

Developers can also reorganize their code to create such a core file, moving startup‑critical functions into a single script. The feature is available starting in Chrome 136, which is rolling out now.

Important: The hint should be used sparingly. “Compiling too many functions consumes time and memory, so only mark the code that really runs on load,” warned Richter.

What This Means for Web Development

Explicit Compile Hints give developers a new, precise lever to optimize JavaScript startup, shifting performance tuning from passive heuristics to active intent. For sites with large JavaScript bundles, it can reduce the gap between “first paint” and “fully interactive.”

“This is a paradigm shift,” said Kim. “Instead of hoping V8 guesses right, developers can now say, ‘This code is important—compile it now.’ It’s a simple but powerful addition to the performance toolbox.”

Combined with other techniques like code splitting, lazy loading, and the use of async/defer, Explicit Compile Hints provide another layer of control. The Chrome team plans to extend the feature in future versions to allow per-function hints.

See It in Action

Developers can observe compile hints working by logging V8’s function events. A minimal test setup:

• index.html with two script tags
• script1.js – a normal file without a hint
• script2.js – with //# allFunctionsCalledOnLoad at the top

Run Chrome with a clean user data directory to avoid interference from code caching. The command line would include --user-data-dir=new and --js-flags="--log-function-events".

For full details, see the official V8 documentation on compile hints.