diff --git a/gtda/diagrams/features.py b/gtda/diagrams/features.py
index 3d10db805..e53531c01 100644
--- a/gtda/diagrams/features.py
+++ b/gtda/diagrams/features.py
@@ -113,7 +113,7 @@ def transform(self, X, y=None):
 
         with np.errstate(divide='ignore', invalid='ignore'):
             Xt = Parallel(n_jobs=self.n_jobs)(
-                delayed(self._persistence_entropy)(_subdiagrams(X, [dim])[s])
+                delayed(self._persistence_entropy)(_subdiagrams(X[s], [dim]))
                 for dim in self.homology_dimensions_
                 for s in gen_even_slices(
                     X.shape[0], effective_n_jobs(self.n_jobs))
diff --git a/gtda/diagrams/representations.py b/gtda/diagrams/representations.py
index 355749aba..16440ae7b 100644
--- a/gtda/diagrams/representations.py
+++ b/gtda/diagrams/representations.py
@@ -588,13 +588,13 @@ def transform(self, X, y=None):
         X = check_diagrams(X, copy=True)
 
         Xt = Parallel(n_jobs=self.n_jobs, mmap_mode='c')(delayed(
-            heats)(_subdiagrams(X, [dim], remove_dim=True)[s],
+            heats)(_subdiagrams(X[s], [dim], remove_dim=True),
                    self._samplings[dim], self._step_size[dim], self.sigma)
             for dim in self.homology_dimensions_
             for s in gen_even_slices(X.shape[0],
                                      effective_n_jobs(self.n_jobs)))
-        Xt = np.concatenate(Xt).reshape(self._n_dimensions, X.shape[0],
-                                        self.n_bins, self.n_bins).\
+        Xt = np.concatenate(Xt).\
+            reshape(self._n_dimensions, X.shape[0], self.n_bins, self.n_bins).\
             transpose((1, 0, 2, 3))
         return Xt
 
@@ -793,18 +793,19 @@ def transform(self, X, y=None):
         X = check_diagrams(X, copy=True)
 
         Xt = Parallel(n_jobs=self.n_jobs, mmap_mode='c')(
-            delayed(persistence_images)(_subdiagrams(X, [dim],
-                                                     remove_dim=True)[s],
-                                        self._samplings[dim],
-                                        self._step_size[dim],
-                                        self.weights_[dim],
-                                        self.sigma)
+            delayed(persistence_images)(
+                _subdiagrams(X[s], [dim], remove_dim=True),
+                self._samplings[dim],
+                self._step_size[dim],
+                self.weights_[dim],
+                self.sigma
+            )
             for dim in self.homology_dimensions_
             for s in gen_even_slices(X.shape[0],
                                      effective_n_jobs(self.n_jobs))
         )
-        Xt = np.concatenate(Xt).reshape(self._n_dimensions, X.shape[0],
-                                        self.n_bins, self.n_bins).\
+        Xt = np.concatenate(Xt).\
+            reshape(self._n_dimensions, X.shape[0], self.n_bins, self.n_bins).\
             transpose((1, 0, 2, 3))
         return Xt
 
@@ -975,7 +976,7 @@ def transform(self, X, y=None):
         X = check_diagrams(X)
 
         Xt = (Parallel(n_jobs=self.n_jobs)
-              (delayed(silhouettes)(_subdiagrams(X, [dim], remove_dim=True)[s],
+              (delayed(silhouettes)(_subdiagrams(X[s], [dim], remove_dim=True),
                                     self._samplings[dim], power=self.power)
               for dim in self.homology_dimensions_
               for s in gen_even_slices(X.shape[0],