diff --git a/scripts/tcd/Basis_Function.py b/scripts/tcd/Basis_Function.py
index 758dc2a32ca782e2cdd5fbb357edbce6387d3e57..35701b09241055fa7e22e2d2149bcf44503df374 100644
--- a/scripts/tcd/Basis_Function.py
+++ b/scripts/tcd/Basis_Function.py
@@ -356,17 +356,14 @@ class OrthonormalLegendre(Legendre):
         ndarray
             Inverse mass matrix.
 
+        Notes
+        -----
+        To increase speed, this function uses that for the orthonormal Legendre
+        polynomial basis the inverse mass matrix is equal to the identity
+        matrix.
+
         """
-        mass_matrix = []
-        for i in range(self._polynomial_degree+1):
-            new_row = []
-            for j in range(self._polynomial_degree+1):
-                new_entry = 0.0
-                if i == j:
-                    new_entry = 1.0
-                new_row.append(new_entry)
-            mass_matrix.append(new_row)
-        return np.array(mass_matrix)
+        return np.identity(self._polynomial_degree+1)
 
     @property
     @cache