diff --git a/graphrag/index/update/entities.py b/graphrag/index/update/entities.py
index 7a74d1b6e0..2057f8f5ae 100644
--- a/graphrag/index/update/entities.py
+++ b/graphrag/index/update/entities.py
@@ -3,8 +3,6 @@
 
 """Entity related operations and utils for Incremental Indexing."""
 
-import itertools
-
 import numpy as np
 import pandas as pd
 
@@ -45,32 +43,69 @@ def _group_and_resolve_entities(
         initial_id, initial_id + len(delta_entities_df)
     )
     # Concat A and B
+    # HACK: Setting ignore_index=False is faster if indexes are already default range, but since old_entities_df and delta_entities_df may have different indexes, we cannot guarantee safety. Leave as is.
     combined = pd.concat(
         [old_entities_df, delta_entities_df], ignore_index=True, copy=False
     )
 
-    # Group by title and resolve conflicts
-    aggregated = (
-        combined.groupby("title")
-        .agg({
-            "id": "first",
-            "type": "first",
-            "human_readable_id": "first",
-            "description": lambda x: list(x.astype(str)),  # Ensure str
-            # Concatenate nd.array into a single list
-            "text_unit_ids": lambda x: list(itertools.chain(*x.tolist())),
-            "degree": "first",  # todo: we could probably re-compute this with the entire new graph
-            "x": "first",
-            "y": "first",
-        })
-        .reset_index()
-    )
+    # ---- FAST LAMBDA REPLACEMENTS IN AGG ----
+    # Precompute grouped "description" and "text_unit_ids" with efficient vectorized routines to avoid slow Python lambdas in groupby-agg
+
+    # Prepare mapping from title (index) to stringified combined descriptions
+    # and title to flattened list of text_unit_ids
+
+    # The columns might have type object (lists), but to ensure we avoid costly .tolist() and .astype(str),
+    # we use .values and numpy operations before aggregation.
+
+    # Build all required columns up front and aggregate using a single pass
+
+    # For "description", convert all to str up front (avoiding .astype(str) for each group in lambda)
+    description_str = combined["description"].astype(str).values
+    # For "text_unit_ids", ensure all are Python lists for chaining
+    text_unit_ids_seq = combined["text_unit_ids"].values
+
+    # Compose the group labels (index per row)
+    titles = combined["title"].values
+    unique_titles, inverse_indices = np.unique(titles, return_inverse=True)
+    n_rows = len(titles)
+    n_groups = len(unique_titles)
+
+    # Group indices for each title
+    index_map = [[] for _ in range(n_groups)]
+    for idx, inv_i in enumerate(inverse_indices):
+        index_map[inv_i].append(idx)
+
+    # Precompute "description" and "text_unit_ids" per group using list comprehensions
+    description_grouped = [[description_str[i] for i in idxs] for idxs in index_map]
+    text_unit_ids_grouped = [
+        [x for i in idxs for x in text_unit_ids_seq[i]] for idxs in index_map
+    ]
+
+    # Helper to efficiently apply 'first' aggregation for simple columns
+    def first_grouped(col):
+        arr = combined[col].values
+        return [arr[idxs[0]] for idxs in index_map]
+
+    # Now construct columns for resultant DataFrame
+    aggregated_dict = {
+        "title": unique_titles,
+        "id": first_grouped("id"),
+        "type": first_grouped("type"),
+        "human_readable_id": first_grouped("human_readable_id"),
+        "description": description_grouped,
+        "text_unit_ids": text_unit_ids_grouped,
+        "degree": first_grouped(
+            "degree"
+        ),  # todo: we could probably re-compute this with the entire new graph
+        "x": first_grouped("x"),
+        "y": first_grouped("y"),
+    }
 
-    # recompute frequency to include new text units
-    aggregated["frequency"] = aggregated["text_unit_ids"].apply(len)
+    # Compute frequency as number of text units per entry
+    aggregated_dict["frequency"] = [len(tu) for tu in text_unit_ids_grouped]
 
-    # Force the result into a DataFrame
-    resolved: pd.DataFrame = pd.DataFrame(aggregated)
+    # Create DataFrame directly—no need for groupby.agg
+    resolved = pd.DataFrame(aggregated_dict)
 
     # Modify column order to keep consistency
     resolved = resolved.loc[:, ENTITIES_FINAL_COLUMNS]