diff --git a/repo2readme/cli/main.py b/repo2readme/cli/main.py
index 8780274..1d51535 100644
--- a/repo2readme/cli/main.py
+++ b/repo2readme/cli/main.py
@@ -5,6 +5,8 @@
 import os
 from repo2readme.utils.tree import generate_tree
 from repo2readme.utils.detect_language import detect_lang
+from concurrent.futures import ThreadPoolExecutor, as_completed
+import threading
 
 
 @click.group()
@@ -131,9 +133,13 @@ def format_size(size_bytes):
 
         summaries = []
         errors = []
-        with Progress() as progress:
-            task = progress.add_task("[cyan]Generating summaries...[/cyan]", total=total_documents)
-            for doc in documents:
+        
+        # Skip summarization if there are no documents
+        if total_documents > 0:
+            summaries_lock = threading.Lock()
+            errors_lock = threading.Lock()
+            
+            def process_document(doc):
                 meta = doc["metadata"]
                 try:
                     lang = detect_lang(meta.get("file_type", "text"))
@@ -142,11 +148,23 @@ def format_size(size_bytes):
                         language=lang,
                         content=doc["content"]
                     )
-            
-                    summaries.append(summary)
+                    with summaries_lock:
+                        summaries.append(summary)
                 except Exception as e:
-                    errors.append(f"Error processing {meta.get('file_path')}: {e}")
-                progress.update(task, advance=1)
+                    with errors_lock:
+                        errors.append(f"Error processing {meta.get('file_path')}: {e}")
+            
+            with Progress() as progress:
+                task = progress.add_task("[cyan]Generating summaries...[/cyan]", total=total_documents)
+                
+                # Limit concurrent workers to avoid overwhelming API providers
+                max_workers = min(2, total_documents)
+                
+                with ThreadPoolExecutor(max_workers=max_workers) as executor:
+                    futures = {executor.submit(process_document, doc): doc for doc in documents}
+                    
+                    for future in as_completed(futures):
+                        progress.update(task, advance=1)
 
         rprint("[cyan]Generating README...[/cyan]")