diff --git a/tests/helpers/load-plugin-modules.mjs b/tests/helpers/load-plugin-modules.mjs
index cb3e5bf..1b2f447 100644
--- a/tests/helpers/load-plugin-modules.mjs
+++ b/tests/helpers/load-plugin-modules.mjs
@@ -192,6 +192,7 @@ export function loadArrangeControllerModule() {
 }
 
 let inlineTextGeometryModulePromise;
+let loadTraceModulePromise;
 
 export function loadInlineTextGeometryModule() {
   inlineTextGeometryModulePromise ??= bundleSource(
@@ -637,3 +638,8 @@ export function loadDocxEmbedLoaderModule() {
   });
   return docxEmbedLoaderModulePromise;
 }
+
+export function loadLoadTraceModule() {
+  loadTraceModulePromise ??= bundleSource("src/loadTrace.ts", "load-trace.cjs").then((outfile) => require(outfile));
+  return loadTraceModulePromise;
+}
diff --git a/tests/load-trace.test.mjs b/tests/load-trace.test.mjs
new file mode 100644
index 0000000..75c0dd1
--- /dev/null
+++ b/tests/load-trace.test.mjs
@@ -0,0 +1,157 @@
+import assert from "node:assert/strict";
+import { test } from "node:test";
+import { loadLoadTraceModule, loadLoggerModule } from "./helpers/load-plugin-modules.mjs";
+
+test("monotonicNow prefers performance.now when available", async () => {
+	const originalPerformance = globalThis.performance;
+	let mockTime = 1000;
+
+	try {
+		globalThis.performance = {
+			now: () => mockTime
+		};
+
+		const { monotonicNow } = await loadLoadTraceModule();
+		assert.equal(monotonicNow(), 1000);
+	} finally {
+		globalThis.performance = originalPerformance;
+	}
+});
+
+test("monotonicNow falls back to Date.now when performance is unavailable", async () => {
+	const originalPerformance = globalThis.performance;
+	const originalDateNow = Date.now;
+
+	try {
+		globalThis.performance = undefined;
+		Date.now = () => 2000;
+
+		const { monotonicNow } = await loadLoadTraceModule();
+
+		assert.equal(monotonicNow(), 2000);
+	} finally {
+		globalThis.performance = originalPerformance;
+		Date.now = originalDateNow;
+	}
+});
+
+test("createLoadTrace records phases with relative timings", async () => {
+	const { configureNativePowerPointDocEditorLogger } = await loadLoggerModule();
+	globalThis.window = {
+		nativePowerPointDocEditorDebugLogging: true,
+		nativePowerPointDocEditorDebugLogs: []
+	};
+	configureNativePowerPointDocEditorLogger(true);
+
+	const originalPerformanceNow = globalThis.performance?.now;
+	let mockTime = 1000;
+	if (!globalThis.performance) {
+		globalThis.performance = {};
+	}
+	globalThis.performance.now = () => mockTime;
+
+	const { createLoadTrace } = await loadLoadTraceModule();
+
+	const originalWarn = console.warn;
+	const originalDebug = console.debug;
+	const warnings = [];
+	const debugs = [];
+	console.warn = (...args) => warnings.push(args);
+	console.debug = (...args) => debugs.push(args);
+
+	try {
+		const trace = createLoadTrace('test-scope', { contextKey: 'contextValue' });
+
+		// Phase 1 (fast phase)
+		mockTime = 1050; // +50ms
+		trace.mark('phase1', { dataKey: 'data1' });
+
+		assert.equal(warnings.length, 1);
+		assert.match(warnings[0][0], /load: test-scope: phase1/);
+		assert.equal(warnings[0][1].sinceStartMs, 50);
+		assert.equal(warnings[0][1].sincePreviousMs, 50);
+
+		assert.equal(debugs.length, 1);
+		assert.match(debugs[0][0], /load: test-scope: phase1/);
+		assert.equal(debugs[0][1].sinceStartMs, 50);
+
+		// Phase 2 (slow phase, triggers warnLog)
+		mockTime = 1300; // +250ms
+		trace.mark('phase2', { dataKey: 'data2' });
+
+		assert.equal(warnings.length, 3); // Two marks + 1 logger warning output
+		assert.match(warnings[1][0], /load: test-scope: phase2/);
+		assert.equal(warnings[1][1].sinceStartMs, 300);
+		assert.equal(warnings[1][1].sincePreviousMs, 250);
+
+		assert.match(warnings[2][0], /load: test-scope: slow phase phase2/);
+		assert.equal(warnings[2][1].sinceStartMs, 300);
+		assert.equal(warnings[2][1].sincePreviousMs, 250);
+
+		assert.equal(debugs.length, 2);
+		assert.match(debugs[1][0], /load: test-scope: phase2/);
+
+		// Finish
+		mockTime = 1400; // +100ms
+		trace.finish('test-finish', { dataKey: 'data3' });
+
+		assert.equal(warnings.length, 4); // 3 marks + 1 logger warning output
+		assert.match(warnings[3][0], /load: test-scope: test-finish/);
+		assert.equal(warnings[3][1].totalMs, 400);
+		assert.equal(warnings[3][1].phases.length, 2);
+		assert.deepEqual(warnings[3][1].phases[0], { phase: 'phase1', sinceStartMs: 50, sincePreviousMs: 50 });
+
+		assert.equal(debugs.length, 3);
+		assert.match(debugs[2][0], /load: test-scope: test-finish/);
+
+	} finally {
+		if (originalPerformanceNow !== undefined) {
+			globalThis.performance.now = originalPerformanceNow;
+		} else {
+			globalThis.performance = undefined;
+		}
+		console.warn = originalWarn;
+		console.debug = originalDebug;
+		globalThis.window = undefined;
+	}
+});
+
+test("createLoadTrace logs warnings for slow loads", async () => {
+	const originalPerformanceNow = globalThis.performance?.now;
+	let mockTime = 1000;
+	if (!globalThis.performance) {
+		globalThis.performance = {};
+	}
+	globalThis.performance.now = () => mockTime;
+
+	const { createLoadTrace } = await loadLoadTraceModule();
+
+	const originalWarn = console.warn;
+	const originalDebug = console.debug;
+	const warnings = [];
+	console.warn = (...args) => warnings.push(args);
+	console.debug = () => {};
+
+	try {
+		const trace = createLoadTrace('slow-scope');
+
+		// Slow load (>1500ms)
+		mockTime = 2600; // +1600ms
+		trace.finish('done');
+
+		assert.equal(warnings.length, 2); // 1 mark + 1 logger warning output
+		assert.match(warnings[0][0], /load: slow-scope: done/);
+
+		assert.match(warnings[1][0], /load: slow-scope: slow load/);
+		assert.equal(warnings[1][1].totalMs, 1600);
+
+	} finally {
+		if (originalPerformanceNow !== undefined) {
+			globalThis.performance.now = originalPerformanceNow;
+		} else {
+			globalThis.performance = undefined;
+		}
+		console.warn = originalWarn;
+		console.debug = originalDebug;
+	}
+});