diff --git a/.github/workflows/pr-validation.yml b/.github/workflows/pr-validation.yml
index 4c46dcef9..74498780c 100644
--- a/.github/workflows/pr-validation.yml
+++ b/.github/workflows/pr-validation.yml
@@ -90,6 +90,9 @@ jobs:
       - name: Run unit tests
         run: pnpm test
 
+      - name: Run slow tests
+        run: pnpm test:slow
+
   run-e2e-tests:
     runs-on: ubuntu-latest
     container:
diff --git a/package.json b/package.json
index 3b55aa3ba..4047a22cc 100644
--- a/package.json
+++ b/package.json
@@ -5,6 +5,8 @@
   "packageManager": "pnpm@10.25.0",
   "scripts": {
     "test": "vitest run",
+    "test:bench": "VITEST_BENCH=true vitest run",
+    "test:slow": "vitest run --root ./packages/super-editor src/tests/editor/node-import-timing.test.js",
     "test:debug": "pnpm --prefix packages/super-editor run test:debug",
     "test:inspect": "NODE_OPTIONS=\"--inspect-brk=9229\" vitest --pool threads --poolOptions.threads.singleThread",
     "test:all": "vitest run",
@@ -73,6 +75,7 @@
     "eslint-import-resolver-typescript": "catalog:",
     "eslint-plugin-import-x": "catalog:",
     "eslint-plugin-jsdoc": "catalog:",
+    "happy-dom": "catalog:",
     "husky": "catalog:",
     "jsdom": "catalog:",
     "lint-staged": "catalog:",
diff --git a/packages/layout-engine/contracts/vitest.config.mjs b/packages/layout-engine/contracts/vitest.config.mjs
index b9bb27496..29bb879cd 100644
--- a/packages/layout-engine/contracts/vitest.config.mjs
+++ b/packages/layout-engine/contracts/vitest.config.mjs
@@ -4,7 +4,8 @@ import baseConfig from '../../../vitest.baseConfig';
 export default defineConfig({
   ...baseConfig,
   test: {
-    environment: 'jsdom',
+    // Use happy-dom for faster tests (set VITEST_DOM=jsdom to use jsdom)
+    environment: process.env.VITEST_DOM || 'happy-dom',
     include: ['src/**/*.test.ts']
   }
 });
diff --git a/packages/layout-engine/layout-bridge/vitest.config.ts b/packages/layout-engine/layout-bridge/vitest.config.ts
index 3e6e378a3..01c02b92b 100644
--- a/packages/layout-engine/layout-bridge/vitest.config.ts
+++ b/packages/layout-engine/layout-bridge/vitest.config.ts
@@ -1,11 +1,14 @@
 import { defineConfig } from 'vitest/config';
 import baseConfig from '../../../vitest.baseConfig';
 
+const includeBench = process.env.VITEST_BENCH === 'true';
+
 export default defineConfig({
   ...baseConfig,
   test: {
     environment: 'node',
-    include: ['test/**/*.test.ts'],
+    include: includeBench ? ['test/**/performance*.test.ts'] : ['test/**/*.test.ts'],
+    exclude: includeBench ? [] : ['test/**/performance*.test.ts'],
     globals: true,
   },
 });
diff --git a/packages/layout-engine/measuring/dom/vitest.config.mjs b/packages/layout-engine/measuring/dom/vitest.config.mjs
index 1a1b03105..eccf6e4da 100644
--- a/packages/layout-engine/measuring/dom/vitest.config.mjs
+++ b/packages/layout-engine/measuring/dom/vitest.config.mjs
@@ -4,7 +4,8 @@ import baseConfig from '../../../../vitest.baseConfig';
 export default defineConfig({
   ...baseConfig,
   test: {
-    environment: 'jsdom',
+    // Use happy-dom for faster tests (set VITEST_DOM=jsdom to use jsdom)
+    environment: process.env.VITEST_DOM || 'happy-dom',
     include: ['src/**/*.test.ts'],
     setupFiles: ['./vitest.setup.ts'],
   },
diff --git a/packages/layout-engine/painters/dom/vitest.config.mjs b/packages/layout-engine/painters/dom/vitest.config.mjs
index 74d39459a..522217db4 100644
--- a/packages/layout-engine/painters/dom/vitest.config.mjs
+++ b/packages/layout-engine/painters/dom/vitest.config.mjs
@@ -4,7 +4,8 @@ import baseConfig from '../../../../vitest.baseConfig';
 export default defineConfig({
   ...baseConfig,
   test: {
-    environment: 'jsdom',
+    // Use happy-dom for faster tests (set VITEST_DOM=jsdom to use jsdom)
+    environment: process.env.VITEST_DOM || 'happy-dom',
     include: ['src/**/*.test.ts'],
   },
 });
diff --git a/packages/layout-engine/pm-adapter/src/integration.test.ts b/packages/layout-engine/pm-adapter/src/integration.test.ts
index 4fd038ca3..b130e95c3 100644
--- a/packages/layout-engine/pm-adapter/src/integration.test.ts
+++ b/packages/layout-engine/pm-adapter/src/integration.test.ts
@@ -547,7 +547,9 @@ describe('PM → FlowBlock → Measure integration', () => {
     const fragment = mount.querySelector('.superdoc-fragment') as HTMLElement;
     const shadingLayer = fragment.querySelector('.superdoc-paragraph-shading') as HTMLElement;
     expect(shadingLayer).toBeTruthy();
-    expect(shadingLayer.style.backgroundColor).toBe('rgb(170, 187, 204)');
+    // Accept both rgb and hex formats (jsdom uses rgb, happy-dom uses hex)
+    const bgColor = shadingLayer.style.backgroundColor.toLowerCase();
+    expect(bgColor === 'rgb(170, 187, 204)' || bgColor === '#aabbcc').toBe(true);
 
     document.body.removeChild(mount);
   });
diff --git a/packages/layout-engine/pm-adapter/vitest.config.mjs b/packages/layout-engine/pm-adapter/vitest.config.mjs
index de74cddeb..09c648b90 100644
--- a/packages/layout-engine/pm-adapter/vitest.config.mjs
+++ b/packages/layout-engine/pm-adapter/vitest.config.mjs
@@ -5,7 +5,8 @@ import baseConfig from '../../../vitest.baseConfig';
 export default defineConfig({
   ...baseConfig,
   test: {
-    environment: 'jsdom',
+    // Use happy-dom for faster tests (set VITEST_DOM=jsdom to use jsdom)
+    environment: process.env.VITEST_DOM || 'happy-dom',
     include: ['src/**/*.test.ts'],
     setupFiles: [resolve(__dirname, './vitest.setup.ts')],
   },
diff --git a/packages/layout-engine/tests/vitest.config.mjs b/packages/layout-engine/tests/vitest.config.mjs
index e8fdfb817..30c9b0189 100644
--- a/packages/layout-engine/tests/vitest.config.mjs
+++ b/packages/layout-engine/tests/vitest.config.mjs
@@ -1,11 +1,17 @@
 import { defineConfig } from 'vitest/config';
 import baseConfig from '../../../vitest.baseConfig';
 
+const includeBench = process.env.VITEST_BENCH === 'true';
+
 export default defineConfig({
   ...baseConfig,
   test: {
-    environment: 'jsdom',
-    include: ['src/**/*.test.ts', 'src/**/*.bench.ts'],
+    // Use happy-dom for faster tests (set VITEST_DOM=jsdom to use jsdom)
+    environment: process.env.VITEST_DOM || 'happy-dom',
+    include: includeBench
+      ? ['src/**/*.bench.ts']
+      : ['src/**/*.test.ts'],
+    exclude: includeBench ? [] : ['src/**/*.bench.ts'],
     setupFiles: ['./vitest.setup.ts'],
     coverage: {
       enabled: false,
diff --git a/packages/super-editor/src/core/commands/insertContent.test.js b/packages/super-editor/src/core/commands/insertContent.test.js
index ed1eea88e..39c1a220d 100644
--- a/packages/super-editor/src/core/commands/insertContent.test.js
+++ b/packages/super-editor/src/core/commands/insertContent.test.js
@@ -166,6 +166,11 @@ describe('insertContent', () => {
 // Integration-style tests that use a real Editor instance to
 // insert markdown/HTML lists and verify exported OOXML has list numbering.
 describe('insertContent (integration) list export', () => {
+  // Cache loaded DOCX data and helpers to avoid repeated file loading
+  let cachedDocxData = null;
+  let helpers = null;
+  let exportHelpers = null;
+
   const getListParagraphs = (result) => {
     const body = result.elements?.find((el) => el.name === 'w:body');
     const paragraphs = (body?.elements || []).filter((el) => el.name === 'w:p');
@@ -189,16 +194,25 @@ describe('insertContent (integration) list export', () => {
     vi.resetModules();
     vi.doUnmock('../helpers/contentProcessor.js');
 
-    const { loadTestDataForEditorTests, initTestEditor } = await import('../../tests/helpers/helpers.js');
-    const { docx, media, mediaFiles, fonts } = await loadTestDataForEditorTests('blank-doc.docx');
-    const { editor } = initTestEditor({ content: docx, media, mediaFiles, fonts, mode: 'docx' });
+    // Cache helpers and DOCX data on first call
+    if (!helpers) {
+      helpers = await import('../../tests/helpers/helpers.js');
+    }
+    if (!cachedDocxData) {
+      cachedDocxData = await helpers.loadTestDataForEditorTests('blank-doc.docx');
+    }
+    if (!exportHelpers) {
+      exportHelpers = await import('../../tests/export/export-helpers/index.js');
+    }
+
+    const { docx, media, mediaFiles, fonts } = cachedDocxData;
+    const { editor } = helpers.initTestEditor({ content: docx, media, mediaFiles, fonts, mode: 'docx' });
     return editor;
   };
 
   const exportFromEditorContent = async (editor) => {
-    const { getExportedResultWithDocContent } = await import('../../tests/export/export-helpers/index.js');
     const content = editor.getJSON().content || [];
-    return await getExportedResultWithDocContent(content);
+    return await exportHelpers.getExportedResultWithDocContent(content);
   };
 
   it('exports ordered list from markdown with numId/ilvl', async () => {
diff --git a/packages/super-editor/src/extensions/table/table.test.js b/packages/super-editor/src/extensions/table/table.test.js
index 165c37a90..ecbbe3e56 100644
--- a/packages/super-editor/src/extensions/table/table.test.js
+++ b/packages/super-editor/src/extensions/table/table.test.js
@@ -1,9 +1,13 @@
-import { describe, it, expect, beforeEach, afterEach } from 'vitest';
+import { describe, it, expect, beforeAll, beforeEach, afterEach } from 'vitest';
 import { EditorState } from 'prosemirror-state';
 import { loadTestDataForEditorTests, initTestEditor } from '@tests/helpers/helpers.js';
 import { createTable } from './tableHelpers/createTable.js';
 import { promises as fs } from 'fs';
 
+// Cache DOCX data to avoid repeated file loading
+let cachedBlankDoc = null;
+let cachedBordersDoc = null;
+
 /**
  * Find the first table position within the provided document.
  * @param {import('prosemirror-model').Node} doc
@@ -29,8 +33,14 @@ describe('Table commands', async () => {
   let templateBlockAttrs;
   let table;
 
+  // Load DOCX data once before all tests
+  beforeAll(async () => {
+    cachedBlankDoc = await loadTestDataForEditorTests('blank-doc.docx');
+    cachedBordersDoc = await loadTestDataForEditorTests('SD-978-remove-table-borders.docx');
+  });
+
   const setupTestTable = async () => {
-    let { docx, media, mediaFiles, fonts } = await loadTestDataForEditorTests('blank-doc.docx');
+    const { docx, media, mediaFiles, fonts } = cachedBlankDoc;
     ({ editor } = initTestEditor({ content: docx, media, mediaFiles, fonts }));
     ({ schema } = editor);
 
@@ -600,7 +610,7 @@ describe('Table commands', async () => {
 
     describe('table imported from docx', async () => {
       beforeEach(async () => {
-        let { docx, media, mediaFiles, fonts } = await loadTestDataForEditorTests('SD-978-remove-table-borders.docx');
+        const { docx, media, mediaFiles, fonts } = cachedBordersDoc;
         ({ editor } = initTestEditor({ content: docx, media, mediaFiles, fonts }));
 
         tablePos = findTablePos(editor.state.doc);
diff --git a/packages/super-editor/vite.config.js b/packages/super-editor/vite.config.js
index e0036dddb..f4a446c5e 100644
--- a/packages/super-editor/vite.config.js
+++ b/packages/super-editor/vite.config.js
@@ -33,6 +33,7 @@ export default defineConfig(({ mode }) => {
       exclude: [
         ...configDefaults.exclude,
         '**/*.spec.js',
+        '**/node-import-timing.test.js', // Slow test, run separately with test:slow
       ],
       coverage: {
         provider: 'v8',
diff --git a/packages/superdoc/vite.config.js b/packages/superdoc/vite.config.js
index 3ad599ea7..70b78dcdc 100644
--- a/packages/superdoc/vite.config.js
+++ b/packages/superdoc/vite.config.js
@@ -94,7 +94,8 @@ export default defineConfig(({ mode, command}) => {
     test: {
       name: projectLabel,
       globals: true,
-      environment: 'jsdom',
+      // Use happy-dom for faster tests (set VITEST_DOM=jsdom to use jsdom)
+      environment: process.env.VITEST_DOM || 'happy-dom',
       retry: 2,
       testTimeout: 20000,
       hookTimeout: 10000,
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index bddb2e276..292b3b4c6 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -313,6 +313,9 @@ importers:
       eslint-plugin-jsdoc:
         specifier: 'catalog:'
         version: 54.7.0(eslint@9.39.1(jiti@2.6.1))
+      happy-dom:
+        specifier: ^20.3.4
+        version: 20.3.4
       husky:
         specifier: 'catalog:'
         version: 9.1.7