diff --git a/betty/app/__init__.py b/betty/app/__init__.py
index e5006e8fe..3dfdf9dc0 100644
--- a/betty/app/__init__.py
+++ b/betty/app/__init__.py
@@ -347,7 +347,7 @@ def concurrency(self) -> int:
         with suppress(KeyError):
             return int(stdos.environ['BETTY_CONCURRENCY'])
         # Assume that any machine that runs Betty has at least two CPU cores.
-        return stdos.cpu_count() or 2
+        return (stdos.cpu_count() or 2) * 4
 
     @property
     def async_concurrency(self) -> int:
diff --git a/betty/task.py b/betty/task.py
index df5665245..bf1d4bc03 100644
--- a/betty/task.py
+++ b/betty/task.py
@@ -11,11 +11,11 @@
 class Context:
     def __init__(self):
         self._claims_lock = threading.Lock()
-        self._claimed_task_ids: MutableSequence[str] = []
+        self._claimed_task_ids: set[str] = set()
 
     def claim(self, task_id: str) -> bool:
         with self._claims_lock:
             if task_id in self._claimed_task_ids:
                 return False
-            self._claimed_task_ids.append(task_id)
-            return True
+            self._claimed_task_ids.add(task_id)
+        return True