diff --git a/data-store/src/main/java/org/lucares/pdb/datastore/internal/DataStore.java b/data-store/src/main/java/org/lucares/pdb/datastore/internal/DataStore.java
index 9384d24..b0c3fd1 100644
--- a/data-store/src/main/java/org/lucares/pdb/datastore/internal/DataStore.java
+++ b/data-store/src/main/java/org/lucares/pdb/datastore/internal/DataStore.java
@@ -5,12 +5,12 @@ import java.nio.file.Path;
 import java.nio.file.Paths;
 import java.util.ArrayList;
 import java.util.Collections;
-import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
 import java.util.SortedSet;
 import java.util.TreeSet;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
 import java.util.stream.Stream;
@@ -47,7 +47,7 @@ public class DataStore {
 
 	private final List<Doc> docIdToDoc = new ArrayList<>();
 
-	private final Map<String, Map<String, IntList>> keyToValueToDocId = new HashMap<>();
+	private final Map<String, Map<String, IntList>> keyToValueToDocId = new ConcurrentHashMap<>();
 
 	private final StringCompressor stringCompressor;
 	private final FolderStorage folderStorage;
@@ -62,7 +62,7 @@ public class DataStore {
 	private void init(final FolderStorage folderStorage) throws IOException {
 
 		final Stream<Path> files = folderStorage.list();
-		files.forEach(path -> {
+		files.parallel().forEach(path -> {
 
 			final String filename = path.getFileName().toString();
 			final Tags tags = toTags(filename);
@@ -78,12 +78,14 @@ public class DataStore {
 		docIdToDoc.add(new Doc(tags, path));
 
 		for (final String key : tags.getKeys()) {
-			final Map<String, IntList> valueToDocIds = keyToValueToDocId.computeIfAbsent(key, k -> new HashMap<>());
+			final Map<String, IntList> valueToDocIds = keyToValueToDocId.computeIfAbsent(key, k -> new ConcurrentHashMap<>());
 
 			final String value = tags.getValue(key);
 
 			final IntList docIds = valueToDocIds.computeIfAbsent(value, v -> new IntList());
-			docIds.add(docId);
+			synchronized (docIds) {				
+				docIds.add(docId);
+			}
 		}
 	}
 
@@ -103,9 +105,10 @@ public class DataStore {
 		}
 		
 		LOGGER.info(
-				"trimming IntLists of index: values {}, {} kB before, {} kB after, difference {} kB, total size: {} kB, took: {} ms",
+				"trimming IntLists of index: values {}, {} kB before, {} kB after, difference {} kB, took: {} ms",
 				totalValues,
-				(totalBeforeTrim * 4) / 1024, (totalAfterTrim * 4) / 1024,
+				(totalBeforeTrim * 4) / 1024, 
+				(totalAfterTrim * 4) / 1024,
 				((totalBeforeTrim - totalAfterTrim) * 4) / 1024,
 				(totalValues * 4) / 1024,
 				(System.nanoTime() - start) / 1_000_000.0);