diff --git a/frontend/src/ts/components/layout/overlays/Banners.tsx b/frontend/src/ts/components/layout/overlays/Banners.tsx
index 60ed03a466a9..7a04b2f30b75 100644
--- a/frontend/src/ts/components/layout/overlays/Banners.tsx
+++ b/frontend/src/ts/components/layout/overlays/Banners.tsx
@@ -12,6 +12,7 @@ import {
 import { setGlobalOffsetTop } from "../../../states/core";
 import { getSnapshot } from "../../../states/snapshot";
 import { cn } from "../../../utils/cn";
+import { isProfilerMode } from "../../../utils/profiler-mode";
 import { Fa } from "../../common/Fa";
 import { showUpdateNameModal } from "../../modals/account-settings/UpdateNameModal";
 
@@ -111,6 +112,14 @@ export function Banners(): JSXElement {
 
   onMount(() => {
     window.addEventListener("resize", debouncedMarginUpdate);
+    if (isProfilerMode()) {
+      addBanner({
+        level: "error",
+        icon: "fas fa-stopwatch",
+        text: "Profiler mode enabled",
+        important: true,
+      });
+    }
   });
 
   onCleanup(() => {
diff --git a/frontend/src/ts/components/modals/DevOptionsModal.tsx b/frontend/src/ts/components/modals/DevOptionsModal.tsx
index 4f78c89a6367..0a4c767a87e3 100644
--- a/frontend/src/ts/components/modals/DevOptionsModal.tsx
+++ b/frontend/src/ts/components/modals/DevOptionsModal.tsx
@@ -21,6 +21,7 @@ import { toggleUserFakeChartData } from "../../test/result";
 import { disableSlowTimerFail } from "../../test/test-timer";
 import { FaSolidIcon } from "../../types/font-awesome";
 import { setMediaQueryDebugLevel } from "../../ui";
+import { isProfilerMode, setProfilerMode } from "../../utils/profiler-mode";
 import { remoteValidation } from "../../utils/remote-validation";
 import { AnimatedModal } from "../common/AnimatedModal";
 import { Button } from "../common/Button";
@@ -171,6 +172,15 @@ export function DevOptionsModal(): JSXElement {
       label: () => "Event Log Viewer",
       onClick: () => showModal("EventLogViewer"),
     },
+    {
+      icon: "fa-stopwatch",
+      label: () => `Profiler Mode (${isProfilerMode() ? "ON" : "OFF"})`,
+      onClick: () => {
+        setProfilerMode(!isProfilerMode());
+        showNoticeNotification("Profiler mode toggled, reloading...");
+        setTimeout(() => location.reload(), 500);
+      },
+    },
   ];
 
   const addDebugInboxItem = (rewardType: "xp" | "badge" | "none"): void => {
diff --git a/frontend/src/ts/cookies.ts b/frontend/src/ts/cookies.ts
index b5a0bc34ef36..ad877ce9a98a 100644
--- a/frontend/src/ts/cookies.ts
+++ b/frontend/src/ts/cookies.ts
@@ -3,6 +3,7 @@ import { createSignal } from "solid-js";
 import { LocalStorageWithSchema } from "./utils/local-storage-with-schema";
 import { activateAnalytics } from "./controllers/analytics-controller";
 import { activateSentry } from "./sentry";
+import { isProfilerMode } from "./utils/profiler-mode";
 
 const AcceptedCookiesSchema = z
   .object({
@@ -39,7 +40,7 @@ export function activateWhatsAccepted(): void {
   if (accepted?.analytics) {
     activateAnalytics();
   }
-  if (accepted?.sentry) {
+  if (accepted?.sentry && !isProfilerMode()) {
     void activateSentry();
   }
 }
diff --git a/frontend/src/ts/dev/signal-tracker.ts b/frontend/src/ts/dev/signal-tracker.ts
index 4f1b6994a2a5..815654200714 100644
--- a/frontend/src/ts/dev/signal-tracker.ts
+++ b/frontend/src/ts/dev/signal-tracker.ts
@@ -1,4 +1,5 @@
 import { createSignal, DEV } from "solid-js";
+import { isProfilerMode } from "../utils/profiler-mode";
 
 export type TrackedSignal = {
   name: string;
@@ -91,7 +92,7 @@ function formatInitialValue(value: unknown): string {
   }
 }
 
-if (DEV) {
+if (DEV && !isProfilerMode()) {
   type NodeInfo = {
     name: string;
     type: "signal" | "store";
diff --git a/frontend/src/ts/test/events/data.ts b/frontend/src/ts/test/events/data.ts
index 78d0fb852930..a7594f6e220c 100644
--- a/frontend/src/ts/test/events/data.ts
+++ b/frontend/src/ts/test/events/data.ts
@@ -9,6 +9,7 @@ import {
   KeydownEventData,
   KeyupEvent,
   KeyupEventData,
+  TestEvent,
   TestEventData,
   TestEventNoMs,
   TestEventType,
@@ -27,19 +28,25 @@ import { isFunboxActiveWithProperty } from "../funbox/list";
 import { getCurrentQuote } from "../../states/test";
 
 export function buildEventLog(): EventLog {
-  return {
-    version: EVENT_LOG_VERSION,
-    events: getAllTestEvents(),
-    context: {
-      targetWords: [...TestWords.words.list],
-      mode: Config.mode,
-      mode2: getMode2(Config, getCurrentQuote()),
+  const context = {
+    targetWords: [...TestWords.words.list],
+    mode: Config.mode,
+    mode2: getMode2(Config, getCurrentQuote()),
+    koreanStatus: koreanStatus,
+    bailedOut: bailedOut,
+    ...(Config.mode === "custom" && {
       customTextLimitMode: CustomText.getLimit().mode,
       customTextLimitValue: CustomText.getLimit().value,
+    }),
+    ...(Config.funbox.length !== 0 && {
       isFunboxWithNospacePropertyActive: isFunboxActiveWithProperty("nospace"),
-      koreanStatus: koreanStatus,
-      bailedOut: bailedOut,
-    },
+    }),
+  };
+
+  return {
+    version: EVENT_LOG_VERSION,
+    events: getAllTestEvents(),
+    context,
   };
 }
 
@@ -272,20 +279,31 @@ export function getAllTestEvents(): TestEventNoMs[] {
   const startEventMs =
     timerEvents.find((e) => e.data.event === "start")?.ms ?? firstEventMs ?? 0;
 
-  // cachedAllEvents = testData300;
-  // return cachedAllEvents;
-  cachedAllEvents = [
-    ...keydownEvents,
-    ...keyupEvents,
-    ...timerEvents,
-    ...inputEvents,
-    ...compositionEvents,
-  ]
+  const total =
+    keydownEvents.length +
+    keyupEvents.length +
+    timerEvents.length +
+    inputEvents.length +
+    compositionEvents.length;
+
+  const merged = new Array<TestEvent>(total);
+  let p = 0;
+  for (const e of keydownEvents) merged[p++] = e;
+  for (const e of keyupEvents) merged[p++] = e;
+  for (const e of timerEvents) merged[p++] = e;
+  for (const e of inputEvents) merged[p++] = e;
+  for (const e of compositionEvents) merged[p++] = e;
+
+  cachedAllEvents = merged
     .sort((a, b) => a.ms - b.ms || sortTieRank(a.type) - sortTieRank(b.type))
-    .map(({ ms, ...rest }) => ({
-      ...rest,
-      testMs: roundTo2(ms - startEventMs),
-    }));
+    .map(
+      (event) =>
+        ({
+          type: event.type,
+          testMs: roundTo2(event.ms - startEventMs),
+          data: event.data,
+        }) as TestEventNoMs,
+    );
 
   return cachedAllEvents;
 }
diff --git a/frontend/src/ts/test/events/types.ts b/frontend/src/ts/test/events/types.ts
index 5095c3ea8353..60566bdbd56a 100644
--- a/frontend/src/ts/test/events/types.ts
+++ b/frontend/src/ts/test/events/types.ts
@@ -129,10 +129,10 @@ export type EventLogContext = {
   // isTimedTest: boolean;
   mode: Config["mode"];
   mode2: ReturnType<typeof getMode2>;
-  customTextLimitMode: CustomTextLimitMode;
-  customTextLimitValue: number;
+  customTextLimitMode?: CustomTextLimitMode;
+  customTextLimitValue?: number;
+  isFunboxWithNospacePropertyActive?: boolean;
   bailedOut: boolean;
-  isFunboxWithNospacePropertyActive: boolean;
   koreanStatus: boolean;
 };
 
diff --git a/frontend/src/ts/test/test-ui.ts b/frontend/src/ts/test/test-ui.ts
index 0e6c155d6b1b..1bc5b2ed6825 100644
--- a/frontend/src/ts/test/test-ui.ts
+++ b/frontend/src/ts/test/test-ui.ts
@@ -1277,16 +1277,22 @@ function buildWordLettersHTML(
     ) {
       extraCorrected = "extraCorrected";
     }
+
+    let displayLetter = inputCharacters[c];
+    if (displayLetter === " ") {
+      displayLetter = "_";
+    }
+
     if (Config.mode === "zen" || wordCharacters[c] !== undefined) {
       if (Config.mode === "zen" || inputCharacters[c] === wordCharacters[c]) {
         if (
           correctedChar === inputCharacters[c] ||
           correctedChar === undefined
         ) {
-          out += `<letter class="correct ${extraCorrected}">${inputCharacters[c]}</letter>`;
+          out += `<letter class="correct ${extraCorrected}">${displayLetter}</letter>`;
         } else {
           out += `<letter class="corrected ${extraCorrected}">${
-            inputCharacters[c]
+            displayLetter
           }</letter>`;
         }
       } else {
@@ -1303,7 +1309,7 @@ function buildWordLettersHTML(
         }
       }
     } else {
-      out += `<letter class="incorrect extra">${inputCharacters[c]}</letter>`;
+      out += `<letter class="incorrect extra">${displayLetter}</letter>`;
     }
   }
   return out;
@@ -1391,7 +1397,10 @@ async function loadWordsHistory(): Promise<boolean> {
       );
     } catch (e) {
       try {
-        for (const char of word) {
+        for (let char of word) {
+          if (char === " ") {
+            char = "_";
+          }
           const letterEl = document.createElement("letter");
           letterEl.textContent = char;
           wordEl.appendChild(letterEl);
diff --git a/frontend/src/ts/utils/logger.ts b/frontend/src/ts/utils/logger.ts
index 1415471d3fe8..db81060e50a3 100644
--- a/frontend/src/ts/utils/logger.ts
+++ b/frontend/src/ts/utils/logger.ts
@@ -1,6 +1,7 @@
 import { z } from "zod";
 import { LocalStorageWithSchema } from "./local-storage-with-schema";
 import { isDevEnvironment } from "./env";
+import { isProfilerMode } from "./profiler-mode";
 
 const nativeLog = console.log;
 const nativeWarn = console.warn;
@@ -14,7 +15,9 @@ const debugLogsLS = new LocalStorageWithSchema({
 
 let debugLogs = debugLogsLS.get();
 
-if (isDevEnvironment()) {
+if (isProfilerMode()) {
+  debugLogs = false;
+} else if (isDevEnvironment()) {
   debugLogs = true;
   debug("Debug logs automatically enabled on localhost");
 }
diff --git a/frontend/src/ts/utils/profiler-mode.ts b/frontend/src/ts/utils/profiler-mode.ts
new file mode 100644
index 000000000000..ca4aeeda667f
--- /dev/null
+++ b/frontend/src/ts/utils/profiler-mode.ts
@@ -0,0 +1,23 @@
+import { z } from "zod";
+import { LocalStorageWithSchema } from "./local-storage-with-schema";
+import { isDevEnvironment } from "./env";
+
+const profilerModeLS = new LocalStorageWithSchema({
+  key: "profilerMode",
+  schema: z.boolean(),
+  fallback: false,
+});
+
+// Resolved at module load: profiler mode disables features that initialise
+// at load (signal tracker hook, sentry init, logger debug filter), so toggling
+// it requires a reload to take effect.
+const active = isDevEnvironment() && profilerModeLS.get();
+
+export function isProfilerMode(): boolean {
+  return active;
+}
+
+export function setProfilerMode(value: boolean): void {
+  if (!isDevEnvironment()) return;
+  profilerModeLS.set(value);
+}