reduce the strain on the underlying API

It seems 20 concurrency limit is putting the backend under strain

app.py

@@ -287,7 +287,7 @@ def start_app():
                 cache_examples="lazy",
             )
     
-    demo.queue(default_concurrency_limit=20)
+    demo.queue(default_concurrency_limit=15)
     
     demo.launch(
         server_name="0.0.0.0",