import type { PerformanceMetrics } from "../types.js";
/**
 * Measure performance metrics for a webpage.
 *
 * Captures Core Web Vitals and other performance indicators
 * using the browser's built-in Performance APIs.
 */
export declare function measurePerformance(url: string): Promise<PerformanceMetrics>;
/**
 * Format performance metrics into a readable summary.
 */
export declare function formatPerformanceReport(metrics: PerformanceMetrics): string;
declare global {
    interface Window {
        __perfMetrics: {
            lcp: number | null;
            cls: number;
            tbt: number;
            fid: number | null;
        };
    }
}
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