diff --git a/index.html b/index.html
index c6a0f11d1..ebb12432d 100644
--- a/index.html
+++ b/index.html
@@ -56,7 +56,6 @@
     <!-- Injected from Server env -->
     <script>
       window.GIT_COMMIT = <%- gitCommit %>;
-      window.INSTANCE_ID = <%- instanceId %>;
       window.ASSET_MANIFEST = <%- assetManifest %>;
       window.BOOTSTRAP_CONFIG = {
         gameEnv: <%- gameEnv %>,
diff --git a/nginx.conf b/nginx.conf
index fbba374f4..1465684f9 100644
--- a/nginx.conf
+++ b/nginx.conf
@@ -304,18 +304,14 @@ server {
     }
 
 
-    # Root location with short Nginx cache but no browser cache
+    # Root location with short shared cache for the app shell
     location = / {
         proxy_pass http://127.0.0.1:3000;
-        
-        # Tell browsers not to cache
-        add_header Cache-Control "no-store, no-cache, must-revalidate, proxy-revalidate";
-        add_header Pragma "no-cache";
-        add_header Expires "0";
-        
-        # But let Nginx cache for 1 second to reduce load
+
+        # Cache the shared app shell briefly at the proxy; browser/CDN policy
+        # comes from the upstream Cache-Control header.
         proxy_cache STATIC;
-        proxy_cache_valid 200 302 1s;
+        proxy_cache_valid 200 302 300s;
         proxy_cache_use_stale error timeout updating http_500 http_502 http_503 http_504;
         proxy_cache_lock on;
         
diff --git a/src/client/Main.ts b/src/client/Main.ts
index 59c70c044..b4ed449af 100644
--- a/src/client/Main.ts
+++ b/src/client/Main.ts
@@ -169,7 +169,6 @@ function updateAccountNavButton(userMeResponse: UserMeResponse | false) {
 declare global {
   interface Window {
     GIT_COMMIT: string;
-    INSTANCE_ID: string;
     turnstile: any;
     adsEnabled: boolean;
     PageOS: {
diff --git a/src/client/Matchmaking.ts b/src/client/Matchmaking.ts
index 735f56141..79ddc3da9 100644
--- a/src/client/Matchmaking.ts
+++ b/src/client/Matchmaking.ts
@@ -13,6 +13,7 @@ import { translateText } from "./Utils";
 
 @customElement("matchmaking-modal")
 export class MatchmakingModal extends BaseModal {
+  private static instanceIdPromise: Promise<string> | null = null;
   private gameCheckInterval: ReturnType<typeof setInterval> | null = null;
   private connectTimeout: ReturnType<typeof setTimeout> | null = null;
   @state() private connected = false;
@@ -87,9 +88,10 @@ export class MatchmakingModal extends BaseModal {
 
   private async connect() {
     const config = await getServerConfigFromClient();
+    const instanceId = await MatchmakingModal.getInstanceId();
 
     this.socket = new WebSocket(
-      `${config.jwtIssuer()}/matchmaking/join?instance_id=${window.INSTANCE_ID}`,
+      `${config.jwtIssuer()}/matchmaking/join?instance_id=${encodeURIComponent(instanceId)}`,
     );
     this.socket.onopen = async () => {
       console.log("Connected to matchmaking server");
@@ -130,6 +132,32 @@ export class MatchmakingModal extends BaseModal {
     };
   }
 
+  private static async getInstanceId(): Promise<string> {
+    MatchmakingModal.instanceIdPromise ??= fetch("/api/instance", {
+      cache: "no-store",
+    })
+      .then(async (response) => {
+        if (!response.ok) {
+          throw new Error(
+            `Failed to load instance id: ${response.status} ${response.statusText}`,
+          );
+        }
+
+        const data = (await response.json()) as { instanceId?: string };
+        if (!data.instanceId) {
+          throw new Error("Missing instance id");
+        }
+
+        return data.instanceId;
+      })
+      .catch((error: unknown) => {
+        MatchmakingModal.instanceIdPromise = null;
+        throw error;
+      });
+
+    return MatchmakingModal.instanceIdPromise;
+  }
+
   protected async onOpen(): Promise<void> {
     const userMe = await getUserMe();
     // Early return if modal was closed during async operation
diff --git a/src/server/GamePreviewRoute.ts b/src/server/GamePreviewRoute.ts
index b2abe2b36..c9f3dd83c 100644
--- a/src/server/GamePreviewRoute.ts
+++ b/src/server/GamePreviewRoute.ts
@@ -15,7 +15,7 @@ import {
   ExternalGameInfoSchema,
 } from "./GamePreviewBuilder";
 import { setNoStoreHeaders } from "./NoStoreHeaders";
-import { renderHtmlContent, setHtmlNoCacheHeaders } from "./RenderHtml";
+import { getAppShellContent, setHtmlNoCacheHeaders } from "./RenderHtml";
 
 const requestOrigin = (req: Request, config: ServerConfig): string => {
   const protoHeader = (req.headers["x-forwarded-proto"] as string) ?? "";
@@ -123,7 +123,7 @@ export function registerGamePreviewRoute(opts: {
       }
 
       if (filePath) {
-        const html = await renderHtmlContent(filePath);
+        const html = await getAppShellContent(filePath);
         const root = parse(html);
         const head = root.querySelector("head");
         if (head) {
diff --git a/src/server/Master.ts b/src/server/Master.ts
index 1adbec277..4390f6e0b 100644
--- a/src/server/Master.ts
+++ b/src/server/Master.ts
@@ -11,7 +11,7 @@ import { logger } from "./Logger";
 import { MapPlaylist } from "./MapPlaylist";
 import { MasterLobbyService } from "./MasterLobbyService";
 import { setNoStoreHeaders } from "./NoStoreHeaders";
-import { renderHtml } from "./RenderHtml";
+import { renderAppShell } from "./RenderHtml";
 import { applyStaticAssetCacheControl } from "./StaticAssetCache";
 
 const config = getServerConfigFromServer();
@@ -28,11 +28,14 @@ const __dirname = path.dirname(__filename);
 
 app.use(express.json());
 
-// Middleware to handle HTML files with EJS templating
+// Serve the shared app shell for the root document.
 app.use(async (req, res, next) => {
   if (req.path === "/") {
     try {
-      await renderHtml(res, path.join(__dirname, "../../static/index.html"));
+      await renderAppShell(
+        res,
+        path.join(__dirname, "../../static/index.html"),
+      );
     } catch (error) {
       log.error("Error rendering index.html:", error);
       res.status(500).send("Internal Server Error");
@@ -148,11 +151,17 @@ app.get("/api/health", (_req, res) => {
   }
 });
 
+app.get("/api/instance", (_req, res) => {
+  res.json({
+    instanceId: process.env.INSTANCE_ID ?? "undefined",
+  });
+});
+
 // SPA fallback route
 app.get("*", async function (_req, res) {
   try {
     const htmlPath = path.join(__dirname, "../../static/index.html");
-    await renderHtml(res, htmlPath);
+    await renderAppShell(res, htmlPath);
   } catch (error) {
     log.error("Error rendering SPA fallback:", error);
     res.status(500).send("Internal Server Error");
diff --git a/src/server/RenderHtml.ts b/src/server/RenderHtml.ts
index 58ca4119a..f7898f3da 100644
--- a/src/server/RenderHtml.ts
+++ b/src/server/RenderHtml.ts
@@ -5,12 +5,16 @@ import { buildAssetUrl } from "../core/AssetUrls";
 import { setNoStoreHeaders } from "./NoStoreHeaders";
 import { getRuntimeAssetManifest } from "./RuntimeAssetManifest";
 
+const APP_SHELL_CACHE_CONTROL =
+  "public, max-age=0, s-maxage=300, stale-while-revalidate=86400";
+
+const appShellContentCache = new Map<string, Promise<string>>();
+
 export async function renderHtmlContent(htmlPath: string): Promise<string> {
   const htmlContent = await fs.readFile(htmlPath, "utf-8");
   const assetManifest = await getRuntimeAssetManifest();
   return ejs.render(htmlContent, {
     gitCommit: JSON.stringify(process.env.GIT_COMMIT ?? "undefined"),
-    instanceId: JSON.stringify(process.env.INSTANCE_ID ?? "undefined"),
     assetManifest: JSON.stringify(assetManifest),
     gameEnv: JSON.stringify(process.env.GAME_ENV ?? "dev"),
     manifestHref: buildAssetUrl("manifest.json", assetManifest),
@@ -25,17 +29,38 @@ export async function renderHtmlContent(htmlPath: string): Promise<string> {
   });
 }
 
+export async function getAppShellContent(htmlPath: string): Promise<string> {
+  let cachedContent = appShellContentCache.get(htmlPath);
+  if (!cachedContent) {
+    cachedContent = renderHtmlContent(htmlPath).catch((error: unknown) => {
+      appShellContentCache.delete(htmlPath);
+      throw error;
+    });
+    appShellContentCache.set(htmlPath, cachedContent);
+  }
+  return cachedContent;
+}
+
+export function clearAppShellContentCache(): void {
+  appShellContentCache.clear();
+}
+
+export function setAppShellCacheHeaders(res: Response): void {
+  res.setHeader("Cache-Control", APP_SHELL_CACHE_CONTROL);
+  res.setHeader("Content-Type", "text/html");
+}
+
 export function setHtmlNoCacheHeaders(res: Response): void {
   setNoStoreHeaders(res);
   res.setHeader("ETag", "");
   res.setHeader("Content-Type", "text/html");
 }
 
-export async function renderHtml(
+export async function renderAppShell(
   res: Response,
   htmlPath: string,
 ): Promise<void> {
-  const rendered = await renderHtmlContent(htmlPath);
-  setHtmlNoCacheHeaders(res);
+  const rendered = await getAppShellContent(htmlPath);
+  setAppShellCacheHeaders(res);
   res.send(rendered);
 }
diff --git a/tests/server/RenderHtml.test.ts b/tests/server/RenderHtml.test.ts
new file mode 100644
index 000000000..d4f600f2c
--- /dev/null
+++ b/tests/server/RenderHtml.test.ts
@@ -0,0 +1,60 @@
+import fs from "fs/promises";
+import os from "os";
+import path from "path";
+import { afterEach, describe, expect, test } from "vitest";
+import {
+  clearAppShellContentCache,
+  getAppShellContent,
+  setAppShellCacheHeaders,
+} from "../../src/server/RenderHtml";
+
+describe("RenderHtml", () => {
+  const originalGitCommit = process.env.GIT_COMMIT;
+  let tempDir: string | null = null;
+
+  afterEach(async () => {
+    process.env.GIT_COMMIT = originalGitCommit;
+    clearAppShellContentCache();
+
+    if (tempDir) {
+      await fs.rm(tempDir, { recursive: true, force: true });
+      tempDir = null;
+    }
+  });
+
+  test("reuses cached app shell content", async () => {
+    tempDir = await fs.mkdtemp(path.join(os.tmpdir(), "render-html-"));
+    const htmlPath = path.join(tempDir, "index.html");
+    await fs.writeFile(
+      htmlPath,
+      "<script>window.GIT_COMMIT = <%- gitCommit %>;</script>",
+      "utf8",
+    );
+
+    process.env.GIT_COMMIT = "first";
+    const first = await getAppShellContent(htmlPath);
+
+    process.env.GIT_COMMIT = "second";
+    const second = await getAppShellContent(htmlPath);
+
+    expect(first).toContain('"first"');
+    expect(second).toBe(first);
+    expect(second).not.toContain('"second"');
+  });
+
+  test("sets shared-cache headers for the app shell", () => {
+    const headers = new Map<string, string>();
+    const response = {
+      setHeader(name: string, value: string) {
+        headers.set(name, value);
+      },
+    } as any;
+
+    setAppShellCacheHeaders(response);
+
+    expect(headers.get("Cache-Control")).toBe(
+      "public, max-age=0, s-maxage=300, stale-while-revalidate=86400",
+    );
+    expect(headers.get("Content-Type")).toBe("text/html");
+  });
+});
diff --git a/vite.config.ts b/vite.config.ts
index eefc073c2..e53544896 100644
--- a/vite.config.ts
+++ b/vite.config.ts
@@ -101,7 +101,6 @@ export default defineConfig(({ mode }) => {
               inject: {
                 data: {
                   gitCommit: JSON.stringify("DEV"),
-                  instanceId: JSON.stringify("DEV_ID"),
                   ...htmlAssetData,
                 },
               },