diff --git a/framework/py/flwr/serverapp/strategy/strategy_utils.py b/framework/py/flwr/serverapp/strategy/strategy_utils.py
index 1053f8b96a41..d2b87a6867b9 100644
--- a/framework/py/flwr/serverapp/strategy/strategy_utils.py
+++ b/framework/py/flwr/serverapp/strategy/strategy_utils.py
@@ -111,26 +111,19 @@ def aggregate_metricrecords(
 ) -> MetricRecord:
     """Perform weighted aggregation all MetricRecords using a specific key."""
     # Retrieve weighting factor from MetricRecord
-    weights: list[float] = []
-    for record in records:
-        # Get the first (and only) MetricRecord in the record
-        metricrecord = next(iter(record.metric_records.values()))
-        # Because replies have been checked for consistency,
-        # we can safely cast the weighting factor to float
-        w = cast(float, metricrecord[weighting_metric_name])
-        weights.append(w)
+    weights: list[float] = [
+        cast(float, next(iter(record.metric_records.values()))[weighting_metric_name])
+        for record in records
+    ]
 
-    # Average
     total_weight = sum(weights)
     weight_factors = [w / total_weight for w in weights]
 
     aggregated_metrics = MetricRecord()
     for record, weight in zip(records, weight_factors):
         for record_item in record.metric_records.values():
-            # aggregate in-place
             for key, value in record_item.items():
                 if key == weighting_metric_name:
-                    # We exclude the weighting key from the aggregated MetricRecord
                     continue
                 if key not in aggregated_metrics:
                     if isinstance(value, list):
@@ -139,14 +132,14 @@ def aggregate_metricrecords(
                         aggregated_metrics[key] = value * weight
                 else:
                     if isinstance(value, list):
-                        current_list = cast(list[float], aggregated_metrics[key])
-                        aggregated_metrics[key] = [
-                            curr + val * weight
-                            for curr, val in zip(current_list, value)
-                        ]
+                        curr_list = cast(list[float], aggregated_metrics[key])
+                        # avoid zip if possible: pre-allocated, update in place
+                        for i, val in enumerate(value):
+                            curr_list[i] += val * weight
                     else:
-                        current_value = cast(float, aggregated_metrics[key])
-                        aggregated_metrics[key] = current_value + value * weight
+                        aggregated_metrics[key] = (
+                            cast(float, aggregated_metrics[key]) + value * weight
+                        )
 
     return aggregated_metrics