diff --git a/.changeset/famous-seals-camp.md b/.changeset/famous-seals-camp.md
new file mode 100644
index 000000000000..b1703ee45ee0
--- /dev/null
+++ b/.changeset/famous-seals-camp.md
@@ -0,0 +1,5 @@
+---
+"@astrojs/sitemap": patch
+---
+
+Improves performance when generating the sitemap data
diff --git a/packages/integrations/sitemap/src/generate-sitemap.ts b/packages/integrations/sitemap/src/generate-sitemap.ts
index b10771ce481a..abd5d2c6ff49 100644
--- a/packages/integrations/sitemap/src/generate-sitemap.ts
+++ b/packages/integrations/sitemap/src/generate-sitemap.ts
@@ -1,51 +1,77 @@
 import type { EnumChangefreq } from 'sitemap';
 import type { SitemapItem, SitemapOptions } from './index.js';
-import { parseUrl } from './utils/parse-url.js';
+import { parseI18nUrl } from './utils/parse-i18n-url.js';
 
 /** Construct sitemap.xml given a set of URLs */
-export function generateSitemap(pages: string[], finalSiteUrl: string, opts: SitemapOptions) {
-	const { changefreq, priority, lastmod: lastmodSrc, i18n } = opts!;
+export function generateSitemap(pages: string[], finalSiteUrl: string, opts?: SitemapOptions) {
+	const { changefreq, priority, lastmod: lastmodSrc, i18n } = opts ?? {};
 	// TODO: find way to respect <link rel="canonical"> URLs here
 	const urls = [...pages];
 	urls.sort((a, b) => a.localeCompare(b, 'en', { numeric: true })); // sort alphabetically so sitemap is same each time
 
 	const lastmod = lastmodSrc?.toISOString();
 
-	const { locales, defaultLocale } = i18n || {};
-	const localeCodes = Object.keys(locales || {});
+	// Parse URLs for i18n matching later
+	const { defaultLocale, locales } = i18n ?? {};
+	let getI18nLinks: GetI18nLinks | undefined;
+	if (defaultLocale && locales) {
+		getI18nLinks = createGetI18nLinks(urls, defaultLocale, locales, finalSiteUrl);
+	}
 
-	const getPath = (url: string) => {
-		const result = parseUrl(url, i18n?.defaultLocale || '', localeCodes, finalSiteUrl);
-		return result?.path;
-	};
-	const getLocale = (url: string) => {
-		const result = parseUrl(url, i18n?.defaultLocale || '', localeCodes, finalSiteUrl);
-		return result?.locale;
-	};
+	const urlData: SitemapItem[] = urls.map((url, i) => ({
+		url,
+		links: getI18nLinks?.(i),
+		lastmod,
+		priority,
+		changefreq: changefreq as EnumChangefreq,
+	}));
+
+	return urlData;
+}
+
+type GetI18nLinks = (urlIndex: number) => SitemapItem['links'] | undefined;
+
+function createGetI18nLinks(
+	urls: string[],
+	defaultLocale: string,
+	locales: Record<string, string>,
+	finalSiteUrl: string
+): GetI18nLinks {
+	// `parsedI18nUrls` will have the same length as `urls`, matching correspondingly
+	const parsedI18nUrls = urls.map((url) => parseI18nUrl(url, defaultLocale, locales, finalSiteUrl));
+	// Cache as multiple i18n URLs with the same path will have the same links
+	const i18nPathToLinksCache = new Map<string, SitemapItem['links']>();
 
-	const urlData: SitemapItem[] = urls.map((url) => {
-		let links;
-		if (defaultLocale && locales) {
-			const currentPath = getPath(url);
-			if (currentPath) {
-				const filtered = urls.filter((subUrl) => getPath(subUrl) === currentPath);
-				if (filtered.length > 1) {
-					links = filtered.map((subUrl) => ({
-						url: subUrl,
-						lang: locales[getLocale(subUrl)!],
-					}));
-				}
+	return (urlIndex) => {
+		const i18nUrl = parsedI18nUrls[urlIndex];
+		if (!i18nUrl) {
+			return undefined;
+		}
+
+		const cached = i18nPathToLinksCache.get(i18nUrl.path);
+		if (cached) {
+			return cached;
+		}
+
+		// Find all URLs with the same path (without the locale part), e.g. /en/foo and /es/foo
+		const links: NonNullable<SitemapItem['links']> = [];
+		for (let i = 0; i < parsedI18nUrls.length; i++) {
+			const parsed = parsedI18nUrls[i];
+			if (parsed?.path === i18nUrl.path) {
+				links.push({
+					url: urls[i],
+					lang: locales[parsed.locale],
+				});
 			}
 		}
 
-		return {
-			url,
-			links,
-			lastmod,
-			priority,
-			changefreq: changefreq as EnumChangefreq,
-		};
-	});
+		// If 0 or 1 (which is itself), return undefined to not create any links.
+		// We also don't need to cache this as we know there's no other URLs that would've match this.
+		if (links.length <= 1) {
+			return undefined;
+		}
 
-	return urlData;
+		i18nPathToLinksCache.set(i18nUrl.path, links);
+		return links;
+	};
 }
diff --git a/packages/integrations/sitemap/src/utils/parse-i18n-url.ts b/packages/integrations/sitemap/src/utils/parse-i18n-url.ts
new file mode 100644
index 000000000000..5b7ebf78550d
--- /dev/null
+++ b/packages/integrations/sitemap/src/utils/parse-i18n-url.ts
@@ -0,0 +1,42 @@
+interface ParsedI18nUrl {
+	locale: string;
+	path: string;
+}
+
+// NOTE: The parameters have been schema-validated with Zod
+export function parseI18nUrl(
+	url: string,
+	defaultLocale: string,
+	locales: Record<string, string>,
+	base: string
+): ParsedI18nUrl | undefined {
+	if (!url.startsWith(base)) {
+		return undefined;
+	}
+
+	let s = url.slice(base.length);
+
+	// Handle root URL
+	if (!s || s === '/') {
+		return { locale: defaultLocale, path: '/' };
+	}
+
+	if (s[0] !== '/') {
+		s = '/' + s;
+	}
+
+	// Get locale from path, e.g.
+	// "/en-US/" -> "en-US"
+	// "/en-US/foo" -> "en-US"
+	const locale = s.split('/')[1];
+	if (locale in locales) {
+		// "/en-US/foo" -> "/foo"
+		let path = s.slice(1 + locale.length);
+		if (!path) {
+			path = '/';
+		}
+		return { locale, path };
+	}
+
+	return { locale: defaultLocale, path: s };
+}
diff --git a/packages/integrations/sitemap/src/utils/parse-url.ts b/packages/integrations/sitemap/src/utils/parse-url.ts
deleted file mode 100644
index f9189cf7d60d..000000000000
--- a/packages/integrations/sitemap/src/utils/parse-url.ts
+++ /dev/null
@@ -1,39 +0,0 @@
-export const parseUrl = (
-	url: string,
-	defaultLocale: string,
-	localeCodes: string[],
-	base: string
-) => {
-	if (
-		!url ||
-		!defaultLocale ||
-		localeCodes.length === 0 ||
-		localeCodes.some((key) => !key) ||
-		!base
-	) {
-		throw new Error('parseUrl: some parameters are empty');
-	}
-	if (url.indexOf(base) !== 0) {
-		return undefined;
-	}
-	let s = url.replace(base, '');
-	if (!s || s === '/') {
-		return { locale: defaultLocale, path: '/' };
-	}
-	if (!s.startsWith('/')) {
-		s = '/' + s;
-	}
-	const a = s.split('/');
-	const locale = a[1];
-	if (localeCodes.some((key) => key === locale)) {
-		let path = a.slice(2).join('/');
-		if (path === '//') {
-			path = '/';
-		}
-		if (path !== '/' && !path.startsWith('/')) {
-			path = '/' + path;
-		}
-		return { locale, path };
-	}
-	return { locale: defaultLocale, path: s };
-};
diff --git a/packages/integrations/sitemap/test/units/generate-sitemap.test.js b/packages/integrations/sitemap/test/units/generate-sitemap.test.js
new file mode 100644
index 000000000000..2af0e42ab8a4
--- /dev/null
+++ b/packages/integrations/sitemap/test/units/generate-sitemap.test.js
@@ -0,0 +1,147 @@
+import assert from 'node:assert/strict';
+import { describe, it } from 'node:test';
+import { generateSitemap } from '../../dist/generate-sitemap.js';
+
+const site = 'http://example.com';
+
+describe('generateSitemap', () => {
+	describe('basic', () => {
+		it('works', () => {
+			const items = generateSitemap(
+				[
+					// All pages
+					`${site}/a`,
+					`${site}/b`,
+					`${site}/c`,
+				],
+				site
+			);
+			assert.equal(items.length, 3);
+			assert.equal(items[0].url, `${site}/a`);
+			assert.equal(items[1].url, `${site}/b`);
+			assert.equal(items[2].url, `${site}/c`);
+		});
+
+		it('sorts the items', () => {
+			const items = generateSitemap(
+				[
+					// All pages
+					`${site}/c`,
+					`${site}/a`,
+					`${site}/b`,
+				],
+				site
+			);
+			assert.equal(items.length, 3);
+			assert.equal(items[0].url, `${site}/a`);
+			assert.equal(items[1].url, `${site}/b`);
+			assert.equal(items[2].url, `${site}/c`);
+		});
+
+		it('sitemap props are passed to items', () => {
+			const now = new Date();
+			const items = generateSitemap(
+				[
+					// All pages
+					`${site}/a`,
+					`${site}/b`,
+					`${site}/c`,
+				],
+				site,
+				{
+					changefreq: 'monthly',
+					lastmod: now,
+					priority: 0.5,
+				}
+			);
+
+			assert.equal(items.length, 3);
+
+			assert.equal(items[0].url, `${site}/a`);
+			assert.equal(items[0].changefreq, 'monthly');
+			assert.equal(items[0].lastmod, now.toISOString());
+			assert.equal(items[0].priority, 0.5);
+
+			assert.equal(items[1].url, `${site}/b`);
+			assert.equal(items[1].changefreq, 'monthly');
+			assert.equal(items[1].lastmod, now.toISOString());
+			assert.equal(items[1].priority, 0.5);
+
+			assert.equal(items[2].url, `${site}/c`);
+			assert.equal(items[2].changefreq, 'monthly');
+			assert.equal(items[2].lastmod, now.toISOString());
+			assert.equal(items[2].priority, 0.5);
+		});
+	});
+
+	describe('i18n', () => {
+		it('works', () => {
+			const items = generateSitemap(
+				[
+					// All pages
+					`${site}/a`,
+					`${site}/b`,
+					`${site}/c`,
+					`${site}/es/a`,
+					`${site}/es/b`,
+					`${site}/es/c`,
+					`${site}/fr/a`,
+					`${site}/fr/b`,
+					// `${site}/fr-CA/c`, (intentionally missing for testing)
+				],
+				site,
+				{
+					i18n: {
+						defaultLocale: 'en',
+						locales: {
+							en: 'en-US',
+							es: 'es-ES',
+							fr: 'fr-CA',
+						},
+					},
+				}
+			);
+
+			assert.equal(items.length, 8);
+
+			const aLinks = [
+				{ url: `${site}/a`, lang: 'en-US' },
+				{ url: `${site}/es/a`, lang: 'es-ES' },
+				{ url: `${site}/fr/a`, lang: 'fr-CA' },
+			];
+			const bLinks = [
+				{ url: `${site}/b`, lang: 'en-US' },
+				{ url: `${site}/es/b`, lang: 'es-ES' },
+				{ url: `${site}/fr/b`, lang: 'fr-CA' },
+			];
+			const cLinks = [
+				{ url: `${site}/c`, lang: 'en-US' },
+				{ url: `${site}/es/c`, lang: 'es-ES' },
+			];
+
+			assert.equal(items[0].url, `${site}/a`);
+			assert.deepEqual(items[0].links, aLinks);
+
+			assert.equal(items[1].url, `${site}/b`);
+			assert.deepEqual(items[1].links, bLinks);
+
+			assert.equal(items[2].url, `${site}/c`);
+			assert.deepEqual(items[2].links, cLinks);
+
+			assert.equal(items[3].url, `${site}/es/a`);
+			assert.deepEqual(items[3].links, aLinks);
+
+			assert.equal(items[4].url, `${site}/es/b`);
+			assert.deepEqual(items[4].links, bLinks);
+
+			assert.equal(items[5].url, `${site}/es/c`);
+			assert.deepEqual(items[5].links, cLinks);
+
+			assert.equal(items[6].url, `${site}/fr/a`);
+			assert.deepEqual(items[6].links, aLinks);
+
+			assert.equal(items[7].url, `${site}/fr/b`);
+			assert.deepEqual(items[7].links, bLinks);
+		});
+	});
+});