diff --git a/team/tests/org.eclipse.compare.tests/src/org/eclipse/compare/tests/AllCompareTests.java b/team/tests/org.eclipse.compare.tests/src/org/eclipse/compare/tests/AllCompareTests.java
index 4778a5e6f2e..3305135b9d3 100644
--- a/team/tests/org.eclipse.compare.tests/src/org/eclipse/compare/tests/AllCompareTests.java
+++ b/team/tests/org.eclipse.compare.tests/src/org/eclipse/compare/tests/AllCompareTests.java
@@ -37,6 +37,7 @@
 	PatchUITest.class,
 	RangeDifferencerThreeWayDiffTest.class,
 	CompareUIPluginTest.class,
+	CompareOpenEfficiencyTest.class,
 	StructureCreatorTest.class,
 	CompareFileRevisionEditorInputTest.class})
 public class AllCompareTests {
diff --git a/team/tests/org.eclipse.compare.tests/src/org/eclipse/compare/tests/CompareOpenEfficiencyTest.java b/team/tests/org.eclipse.compare.tests/src/org/eclipse/compare/tests/CompareOpenEfficiencyTest.java
new file mode 100644
index 00000000000..4e004283670
--- /dev/null
+++ b/team/tests/org.eclipse.compare.tests/src/org/eclipse/compare/tests/CompareOpenEfficiencyTest.java
@@ -0,0 +1,304 @@
+/*******************************************************************************
+ * Copyright (c) 2026 Lars Vogel and others.
+ *
+ * This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License 2.0
+ * which accompanies this distribution, and is available at
+ * https://www.eclipse.org/legal/epl-2.0/
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ *
+ * Contributors:
+ *     Eclipse contributors - initial API and implementation
+ *******************************************************************************/
+package org.eclipse.compare.tests;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.junit.jupiter.api.Assertions.fail;
+
+import java.io.ByteArrayInputStream;
+import java.io.InputStream;
+import java.nio.charset.StandardCharsets;
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.function.BooleanSupplier;
+
+import org.eclipse.compare.CompareConfiguration;
+import org.eclipse.compare.CompareEditorInput;
+import org.eclipse.compare.CompareUI;
+import org.eclipse.compare.CompareViewerSwitchingPane;
+import org.eclipse.compare.IEncodedStreamContentAccessor;
+import org.eclipse.compare.ITypedElement;
+import org.eclipse.compare.internal.ComparePreferencePage;
+import org.eclipse.compare.internal.CompareUIPlugin;
+import org.eclipse.compare.internal.IMergeViewerTestAdapter;
+import org.eclipse.compare.structuremergeviewer.DiffNode;
+import org.eclipse.core.runtime.Adapters;
+import org.eclipse.core.runtime.IProgressMonitor;
+import org.eclipse.jface.preference.IPreferenceStore;
+import org.eclipse.jface.viewers.Viewer;
+import org.eclipse.swt.graphics.Image;
+import org.eclipse.swt.widgets.Display;
+import org.eclipse.ui.IWorkbenchPage;
+import org.eclipse.ui.PlatformUI;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Tests how often the compare editor open path re-reads element content and
+ * runs {@code prepareInput}, and on which thread. The asserted bounds are the
+ * current baseline and tighten as the open path is optimized.
+ */
+public class CompareOpenEfficiencyTest {
+
+	/**
+	 * Upper bound for the {@code getContents()} calls per side during a single
+	 * compare editor open, caused by repeated content-type sniffing and viewer
+	 * descriptor lookups. The exact count is platform dependent (observed: 15 on
+	 * Linux and Windows, 9 on macOS), so only the worst case is asserted.
+	 */
+	private static final int MAX_GET_CONTENTS_PER_SIDE = 15;
+
+	private static final long TIMEOUT_MILLIS = 30_000;
+
+	private boolean originalUnifiedDiff;
+
+	/** A text element that counts every {@link #getContents()} call. */
+	private static final class CountingElement
+			implements ITypedElement, IEncodedStreamContentAccessor {
+
+		private final String name;
+		private final byte[] bytes;
+		private final AtomicInteger contentReads = new AtomicInteger();
+
+		CountingElement(String name, String content) {
+			this.name = name;
+			this.bytes = content.getBytes(StandardCharsets.UTF_8);
+		}
+
+		@Override
+		public InputStream getContents() {
+			contentReads.incrementAndGet();
+			return new ByteArrayInputStream(bytes);
+		}
+
+		@Override
+		public String getCharset() {
+			return "UTF-8"; //$NON-NLS-1$
+		}
+
+		@Override
+		public String getName() {
+			return name;
+		}
+
+		@Override
+		public String getType() {
+			return TEXT_TYPE;
+		}
+
+		@Override
+		public Image getImage() {
+			return null;
+		}
+
+		int reads() {
+			return contentReads.get();
+		}
+	}
+
+	/**
+	 * A compare input that counts {@code prepareInput} invocations and records
+	 * whether they ran on the UI thread.
+	 */
+	private static final class CountingCompareEditorInput extends CompareEditorInput {
+
+		private final ITypedElement left;
+		private final ITypedElement right;
+		private final boolean runAsJob;
+		private final AtomicInteger prepareInputCount = new AtomicInteger();
+		private volatile boolean anyRunOnUiThread;
+
+		CountingCompareEditorInput(boolean runAsJob, ITypedElement left, ITypedElement right) {
+			super(new CompareConfiguration());
+			this.runAsJob = runAsJob;
+			this.left = left;
+			this.right = right;
+			setTitle("Counting compare"); //$NON-NLS-1$
+		}
+
+		@Override
+		protected Object prepareInput(IProgressMonitor monitor) {
+			prepareInputCount.incrementAndGet();
+			if (Display.getCurrent() != null) {
+				anyRunOnUiThread = true;
+			}
+			return new DiffNode(left, right);
+		}
+
+		@Override
+		public boolean canRunAsJob() {
+			return runAsJob;
+		}
+
+		int prepareInputCalls() {
+			return prepareInputCount.get();
+		}
+
+		boolean ranOnUiThread() {
+			return anyRunOnUiThread;
+		}
+	}
+
+	@BeforeEach
+	public void setUp() {
+		assertNotNull(Display.getCurrent(), "tests require a UI thread / Display"); //$NON-NLS-1$
+		originalUnifiedDiff = store().getBoolean(ComparePreferencePage.UNIFIED_DIFF);
+	}
+
+	@AfterEach
+	public void tearDown() {
+		store().setValue(ComparePreferencePage.UNIFIED_DIFF, originalUnifiedDiff);
+		IWorkbenchPage page = activePage();
+		if (page != null) {
+			page.closeAllEditors(false);
+		}
+		processQueuedEvents();
+	}
+
+	@Test
+	public void testGetContentsCallCountPerSide() throws Exception {
+		store().setValue(ComparePreferencePage.UNIFIED_DIFF, false);
+		CountingElement leftElement = new CountingElement("left.txt", //$NON-NLS-1$
+				"alpha\nbravo\ncharlie\ndelta\n"); //$NON-NLS-1$
+		CountingElement rightElement = new CountingElement("right.txt", //$NON-NLS-1$
+				"alpha\nBRAVO\ncharlie\nDELTA\n"); //$NON-NLS-1$
+		CountingCompareEditorInput input = new CountingCompareEditorInput(true, leftElement, rightElement);
+
+		CompareUI.openCompareEditor(input);
+		pumpUntil(() -> {
+			IMergeViewerTestAdapter adapter = mergeViewerAdapter(input);
+			return adapter != null && adapter.getChangesCount() > 0;
+		}, "merge viewer did not report a diff"); //$NON-NLS-1$
+
+		System.out.println("PERF-EFFICIENCY getContents left=" + leftElement.reads() //$NON-NLS-1$
+				+ " right=" + rightElement.reads()); //$NON-NLS-1$
+
+		assertReadsWithinBound("left", leftElement.reads()); //$NON-NLS-1$
+		assertReadsWithinBound("right", rightElement.reads()); //$NON-NLS-1$
+	}
+
+	private static void assertReadsWithinBound(String side, int reads) {
+		assertTrue(reads >= 1, "expected at least one getContents() call on the " + side + " side"); //$NON-NLS-1$ //$NON-NLS-2$
+		assertTrue(reads <= MAX_GET_CONTENTS_PER_SIDE,
+				"getContents() called " + reads + " times on the " + side //$NON-NLS-1$ //$NON-NLS-2$
+						+ " side, exceeding the known worst case of " + MAX_GET_CONTENTS_PER_SIDE); //$NON-NLS-1$
+	}
+
+	@Test
+	public void testPrepareInputRunsOnceUnifiedOff() throws Exception {
+		store().setValue(ComparePreferencePage.UNIFIED_DIFF, false);
+		CountingCompareEditorInput input = openAndWait(true);
+		assertEquals(1, input.prepareInputCalls(),
+				"prepareInput must run exactly once per open (unified diff off)"); //$NON-NLS-1$
+	}
+
+	@Test
+	public void testPrepareInputRunsOnceUnifiedOn() throws Exception {
+		store().setValue(ComparePreferencePage.UNIFIED_DIFF, true);
+		CountingCompareEditorInput input = openAndWait(true);
+		// The unified diff path runs the input once in canShowInUnifiedDiff and the
+		// classic fallback reuses the cached result, so the total stays at one.
+		assertEquals(1, input.prepareInputCalls(),
+				"prepareInput invocation count with unified diff on (documents current behavior)"); //$NON-NLS-1$
+	}
+
+	@Test
+	public void testPrepareInputOffUiThreadUnifiedOff() throws Exception {
+		store().setValue(ComparePreferencePage.UNIFIED_DIFF, false);
+		CountingCompareEditorInput input = openAndWait(true);
+		assertTrue(input.prepareInputCalls() >= 1, "prepareInput did not run"); //$NON-NLS-1$
+		assertEquals(false, input.ranOnUiThread(),
+				"prepareInput must run off the UI thread when the input can run as a job"); //$NON-NLS-1$
+	}
+
+	// Documents current behavior: the unified diff path prepares the input on the
+	// UI thread. This assertion flips once the preparation moves to a background job.
+	@Test
+	public void testPrepareInputOnUiThreadUnifiedOnDocumentsCurrentBehavior() throws Exception {
+		store().setValue(ComparePreferencePage.UNIFIED_DIFF, true);
+		CountingCompareEditorInput input = openAndWait(true);
+		assertTrue(input.prepareInputCalls() >= 1, "prepareInput did not run"); //$NON-NLS-1$
+		assertEquals(true, input.ranOnUiThread(),
+				"documents current behavior: unified diff runs prepareInput on the UI thread (Phase 1 item 3)"); //$NON-NLS-1$
+	}
+
+	private CountingCompareEditorInput openAndWait(boolean runAsJob) throws Exception {
+		CountingCompareEditorInput input = new CountingCompareEditorInput(runAsJob,
+				new CountingElement("left.txt", "alpha\nbravo\ncharlie\n"), //$NON-NLS-1$ //$NON-NLS-2$
+				new CountingElement("right.txt", "alpha\nBRAVO\ncharlie\n")); //$NON-NLS-1$ //$NON-NLS-2$
+		CompareUI.openCompareEditor(input);
+		pumpUntil(() -> input.getCompareResult() != null && contentPane(input) != null,
+				"compare editor did not finish opening"); //$NON-NLS-1$
+		return input;
+	}
+
+	private static IMergeViewerTestAdapter mergeViewerAdapter(CompareEditorInput input) {
+		CompareViewerSwitchingPane pane = contentPane(input);
+		if (pane == null) {
+			return null;
+		}
+		Viewer viewer = pane.getViewer();
+		if (viewer == null) {
+			return null;
+		}
+		return Adapters.adapt(viewer, IMergeViewerTestAdapter.class);
+	}
+
+	private static CompareViewerSwitchingPane contentPane(CompareEditorInput input) {
+		try {
+			return (CompareViewerSwitchingPane) ReflectionUtils.getField(input, "fContentInputPane", true); //$NON-NLS-1$
+		} catch (ReflectiveOperationException e) {
+			throw new IllegalStateException(e);
+		}
+	}
+
+	private static void pumpUntil(BooleanSupplier condition, String failMessage) {
+		Display display = Display.getCurrent();
+		long deadline = System.currentTimeMillis() + TIMEOUT_MILLIS;
+		// A self-rescheduling timer keeps the loop waking so the deadline is
+		// enforced even while blocked in Display.sleep().
+		Runnable[] wake = new Runnable[1];
+		wake[0] = () -> display.timerExec(50, wake[0]);
+		display.timerExec(50, wake[0]);
+		try {
+			while (!condition.getAsBoolean()) {
+				if (System.currentTimeMillis() > deadline) {
+					fail(failMessage + " within " + TIMEOUT_MILLIS + "ms"); //$NON-NLS-1$ //$NON-NLS-2$
+				}
+				if (!display.readAndDispatch()) {
+					display.sleep();
+				}
+			}
+		} finally {
+			display.timerExec(-1, wake[0]);
+		}
+	}
+
+	private void processQueuedEvents() {
+		Display display = Display.getCurrent();
+		while (display.readAndDispatch()) {
+			// drain the event queue
+		}
+	}
+
+	private static IWorkbenchPage activePage() {
+		return PlatformUI.getWorkbench().getActiveWorkbenchWindow().getActivePage();
+	}
+
+	private static IPreferenceStore store() {
+		return CompareUIPlugin.getDefault().getPreferenceStore();
+	}
+}