Core/Dash Dimension: Browser

Dimension: Page & Navigation: URLs (u)

The Browser dimension groups performance data based on the User Agent string sent by the client. This allows you to audit Core Web Vitals performance through the lens of the specific software rendering your application (e.g., Chrome, Firefox, Safari, Edge, Samsung Internet).

The Browser dimension isolates software constraints, rendering engine differences (Blink, Gecko, WebKit) and third-party script compatibility.

RUM vs. CrUX

Understanding the source of your data is important for accurate engineering analysis.

• CrUX (Chrome User Experience Report): This dataset collects data exclusively from opt-in users on Chrome (and some Chromium derivatives). It blindly excludes traffic from Firefox (Gecko engine) and Safari (WebKit engine).
• CoreDash RUM: Collects data from every browser that executes the JavaScript snippet.

For many websites, non-Chrome browsers represent 30-50% of traffic. Relying solely on CrUX creates a  blind spot: you are optimizing for Google's V8 engine while neglecting the SpiderMonkey (Firefox) and JavaScriptCore (Safari) engines used by a massive segment of your audience..

Metric-Specific Diagnostics

Different browser engines manage resources, compile JavaScript, and calculate layout geometry differently. Use this dimension to pinpoint engine-specific failures.

Interaction to Next Paint (INP)

INP issues directly correlate with the efficiency of the browser's JavaScript engine and main-thread scheduling.

• Firefox (SpiderMonkey): Firefox handles task prioritization differently than Chrome. A heavy event listener that passes in Chrome might cause noticeable input delay in Firefox due to differences in how the main thread yields.
• Safari (JavaScriptCore): often exhibits distinct behaviors regarding "tap" latency on mobile. Hydration logic that feels instant on Android (Chrome) may trigger delays on iOS due to distinct event propagation models.

Largest Contentful Paint (LCP)

LCP discrepancies usually signal a lack of feature parity or support for modern optimization APIs.

• Format Negotiation: If Chrome reports a fast LCP but Firefox lags, verify your image format strategy. You may be serving AVIF to Chrome while falling back to larger JPEGs for older browser versions that lack support.
• Priority Hints: Chrome aggressively respects fetchpriority="high". Browsers that ignore this attribute treat the LCP resource with standard priority, artificially inflating the Load Delay.
• Connection Protocols: Edge and Firefox may negotiate HTTP/3 (QUIC) connections differently in corporate or restricted network environments, impacting the TTFB component of LCP.

Cumulative Layout Shift (CLS)

Rendering engines calculate pixel geometry using distinct sub-pixel logic.

• Font Rendering (Gecko vs. Blink): Firefox (Gecko) and Chrome (Blink) render font baselines and tracking slightly differently. If you do not match your fallback font metrics perfectly, the text block will resize when the web font loads, causing a shift in one browser but not the other.
• Scrollbar Reservation: Windows browsers (Edge/Firefox/Chrome) reserve physical space for scrollbars, whereas macOS browsers overlay them. This disparity often causes width-based layout shifts that are invisible during development on a Mac but prominent for Windows users.

Workflow: Isolating Engine-Specific Regressions

The primary use case for this dimension is "Engine Validation."

• Identify the Outlier: Sort your Browser table by Impact or Volume. Look for a specific browser (e.g., Firefox Mobile) that has a significantly worse score than the baseline (Chrome Mobile).
• Verify the Environment: Check if the issue is strictly related to the browser or a combination of browser and OS (e.g., Edge on Android vs. Edge on Windows).
• Debug: If Edge is slow but Chrome is fast (both use Blink), investigate third-party extensions or enterprise security software common to Edge users that inject scripts into the DOM.If Firefox is slow, audit your CSS for non-standard properties or layout thrashing that Gecko penalizes more heavily than Blink.

Legacy and Embedded Browsers

Use the Browser dimension to identify "Long Tail" performance drags.

In-App Browsers: Traffic from Instagram, LinkedIn, or Facebook often runs in restricted WebViews that behave differently than the native mobile browser.

Legacy Versions: You may find traffic from outdated browser versions. If these generate high INP, configure your build tools (Babel/PostCSS) to either serve necessary polyfills or,if the volume is negligible, make the strategic decision to drop support to reduce the bundle size for modern users.