diff --git a/core/org.eclipse.cdt.core/.options b/core/org.eclipse.cdt.core/.options
index c5ba283ae3b..150f80e521c 100644
--- a/core/org.eclipse.cdt.core/.options
+++ b/core/org.eclipse.cdt.core/.options
@@ -33,6 +33,9 @@ org.eclipse.cdt.core/debug/typeresolver=false
 # Reports issues with locking the index
 org.eclipse.cdt.core/debug/index/locks=false
 
+# Reports unusually slow IndexFileSet operations
+org.eclipse.cdt.core/debug/index/indexfileset=false
+
 # Reports events related to setting up the indexer for a project
 org.eclipse.cdt.core/debug/indexer/setup=false
 
diff --git a/core/org.eclipse.cdt.core/parser/org/eclipse/cdt/internal/core/index/IndexFileSet.java b/core/org.eclipse.cdt.core/parser/org/eclipse/cdt/internal/core/index/IndexFileSet.java
index b5cf0b4e2b5..4a8a92036dd 100644
--- a/core/org.eclipse.cdt.core/parser/org/eclipse/cdt/internal/core/index/IndexFileSet.java
+++ b/core/org.eclipse.cdt.core/parser/org/eclipse/cdt/internal/core/index/IndexFileSet.java
@@ -30,6 +30,8 @@ import org.eclipse.core.runtime.Assert;
 import org.eclipse.core.runtime.CoreException;
 
 public class IndexFileSet implements IIndexFileSet {
+	public static boolean sDEBUG_INDEX_FILE_SET; // Initialized in the PDOMManager.
+
 	private IIndexFileSet fInverse;
 	private final HashMap<IIndexFragment, IIndexFragmentFileSet> fSubSets= new HashMap<>();
 	private final Map<IBinding, Boolean> fDeclarationContainmentCache = new HashMap<>();
@@ -67,6 +69,7 @@ public class IndexFileSet implements IIndexFileSet {
 		if (cachedValue != null)
 			return cachedValue;
 
+		int iterationCount = 0;
 		for (Map.Entry<IIndexFragment, IIndexFragmentFileSet> entry : fSubSets.entrySet()) {
 			IIndexFragment fragment = entry.getKey();
 			IIndexFragmentFileSet fragmentFileSet = entry.getValue();
@@ -81,14 +84,35 @@ public class IndexFileSet implements IIndexFileSet {
 						long fileRecord = PDOMName.getFileRecord(db, nameRecord);
 						if (pdomFileSet.containsFile(fileRecord)) {
 							fDeclarationContainmentCache.put(binding, true);
+							if (sDEBUG_INDEX_FILE_SET && iterationCount >= 200) {
+								System.out.println(
+										String.format("IndexFileSet: %s (%s) found after %d iterations", //$NON-NLS-1$
+												String.join("::", binding.getQualifiedName()), //$NON-NLS-1$
+												binding.getClass().getSimpleName(),
+												iterationCount));
+							}
 							return true;
 						}
+						if (sDEBUG_INDEX_FILE_SET && ++iterationCount % 1000 == 0) {
+							System.out.println(
+									String.format("IndexFileSet: %s (%s) not yet found after %d iterations", //$NON-NLS-1$
+											String.join("::", binding.getQualifiedName()), //$NON-NLS-1$
+											binding.getClass().getSimpleName(),
+											iterationCount));
+						}
 					}
 				}
 			} catch (CoreException e) {
 				CCorePlugin.log(e);
 			}
 		}
+		if (sDEBUG_INDEX_FILE_SET && iterationCount >= 200) {
+			System.out.println(
+					String.format("IndexFileSet: %s (%s) not found after %d iterations", //$NON-NLS-1$
+							String.join("::", binding.getQualifiedName()), //$NON-NLS-1$
+							binding.getClass().getSimpleName(),
+							iterationCount));
+		}
 		fDeclarationContainmentCache.put(binding, false);
 		return false;
 	}
diff --git a/core/org.eclipse.cdt.core/parser/org/eclipse/cdt/internal/core/pdom/PDOM.java b/core/org.eclipse.cdt.core/parser/org/eclipse/cdt/internal/core/pdom/PDOM.java
index 3808bc5bfbc..8c59bcc439d 100644
--- a/core/org.eclipse.cdt.core/parser/org/eclipse/cdt/internal/core/pdom/PDOM.java
+++ b/core/org.eclipse.cdt.core/parser/org/eclipse/cdt/internal/core/pdom/PDOM.java
@@ -110,7 +110,7 @@ public class PDOM extends PlatformObject implements IPDOM {
 	private static final int BLOCKED_WRITE_LOCK_OUTPUT_INTERVAL = 30000;
 	private static final int LONG_WRITE_LOCK_REPORT_THRESHOLD = 1000;
 	private static final int LONG_READ_LOCK_WAIT_REPORT_THRESHOLD = 1000;
-	static boolean sDEBUG_LOCKS= false; // Initialized in the PDOMManager, because IBM needs PDOM independent of runtime plugin.
+	static boolean sDEBUG_LOCKS; // Initialized in the PDOMManager, because IBM needs PDOM independent of runtime plugin.
 
 	/**
 	 * Identifier for PDOM format
diff --git a/core/org.eclipse.cdt.core/parser/org/eclipse/cdt/internal/core/pdom/PDOMManager.java b/core/org.eclipse.cdt.core/parser/org/eclipse/cdt/internal/core/pdom/PDOMManager.java
index 02ac75d1194..056607eb636 100644
--- a/core/org.eclipse.cdt.core/parser/org/eclipse/cdt/internal/core/pdom/PDOMManager.java
+++ b/core/org.eclipse.cdt.core/parser/org/eclipse/cdt/internal/core/pdom/PDOMManager.java
@@ -69,6 +69,7 @@ import org.eclipse.cdt.internal.core.index.IWritableIndex;
 import org.eclipse.cdt.internal.core.index.IWritableIndexManager;
 import org.eclipse.cdt.internal.core.index.IndexChangeEvent;
 import org.eclipse.cdt.internal.core.index.IndexFactory;
+import org.eclipse.cdt.internal.core.index.IndexFileSet;
 import org.eclipse.cdt.internal.core.index.IndexerStateEvent;
 import org.eclipse.cdt.internal.core.index.provider.IndexProviderManager;
 import org.eclipse.cdt.internal.core.pdom.PDOM.IListener;
@@ -198,6 +199,8 @@ public class PDOMManager implements IWritableIndexManager, IListener {
 
 	public PDOMManager() {
 		PDOM.sDEBUG_LOCKS= Boolean.parseBoolean(Platform.getDebugOption(CCorePlugin.PLUGIN_ID + "/debug/index/locks"));  //$NON-NLS-1$
+		IndexFileSet.sDEBUG_INDEX_FILE_SET=
+				Boolean.parseBoolean(Platform.getDebugOption(CCorePlugin.PLUGIN_ID + "/debug/index/indexfileset"));  //$NON-NLS-1$
 		addIndexerSetupParticipant(new WaitForRefreshJobs());
 		fProjectDescriptionListener= new CProjectDescriptionListener(this);
 		fJobChangeListener= new JobChangeListener(this);