diff --git a/.gitignore b/.gitignore
index 5a2dce5..c59b0c5 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,5 @@
 .gitignore
 /venv
-/.pytest_cache
\ No newline at end of file
+/.pytest_cache
+__pycache__/
+*.pyc
diff --git a/processing.py b/processing.py
index 5acd427..80e1ce0 100644
--- a/processing.py
+++ b/processing.py
@@ -98,16 +98,26 @@ def map_psms_to_spectra(spectra: List[Dict], psm_df: pd.DataFrame) -> pd.DataFra
 
     # ⚡ OPTIMIZATION: Convert list of dicts to DataFrame directly instead of repeated apply calls
     # Original: Multiple apply calls (4x iteration over full dataset)
+    # Improvement: Iterate over list of lists (faster than dicts) and explicitly pass columns.
+    # This also ensures 'None' is used for missing values instead of 'NaN'.
 
-    # Convert matched Series to list, replacing NaNs with empty dicts for DataFrame construction
-    specs_list = [x if isinstance(x, dict) else {} for x in matched_spec_series]
-    specs_df = pd.DataFrame(specs_list)
-    specs_df.index = psm_df.index  # Align index with original DataFrame
+    # Pre-define columns to ensure order and existence
+    cols = ['title', 'mz_array', 'intensity_array', 'pepmass']
+
+    # Convert Series to list to avoid Pandas iteration overhead (~2x speedup)
+    matched_list = matched_spec_series.tolist()
 
-    # Ensure required columns exist (if no spectra matched or mock data missing keys)
-    for col in ['title', 'mz_array', 'intensity_array', 'pepmass']:
-        if col not in specs_df.columns:
-            specs_df[col] = None
+    specs_data = []
+    for x in matched_list:
+        if isinstance(x, dict):
+            # Direct access for speed (keys guaranteed by load_mgf)
+            specs_data.append([x['title'], x['mz_array'], x['intensity_array'], x['pepmass']])
+        else:
+            # Explicitly use None for missing matches
+            specs_data.append([None, None, None, None])
+
+    specs_df = pd.DataFrame(specs_data, columns=cols)
+    specs_df.index = psm_df.index  # Align index with original DataFrame
 
     mappings = pd.DataFrame({
         'psm_index': psm_df.index,