time-read.diff

diff --git a/sphinx/builders/__init__.py b/sphinx/builders/__init__.py
index 8eaa0e215..62887098a 100644
--- a/sphinx/builders/__init__.py
+++ b/sphinx/builders/__init__.py
@@ -452,12 +452,33 @@ class Builder:
 
     def _read_serial(self, docnames):
         # type: (List[str]) -> None
+        times = []
+        import psutil
+        p = psutil.Process()
+        refmem = p.memory_info().rss
+
         for docname in status_iterator(docnames, __('reading sources... '), "purple",
                                        len(docnames), self.app.verbosity):
             # remove all inventory entries for that file
             self.events.emit('env-purge-doc', self.env, docname)
             self.env.clear_doc(docname)
+            t0 = time.time()
             self.read_doc(docname)
+            mem = p.memory_info().rss
+            times.append((time.time() - t0, docname, (mem - refmem) / 1e6))
+            refmem = mem
+            print(times[-1])
+
+        times.sort(reverse=True)
+        print('50 slowest tests:')
+        for t, name, mem in times[:50]:
+            print(f'{docname} : {t:.2f} sec., {mem:.2f} Mb')
+
+        import operator
+        times.sort(key=operator.itemgetter(2), reverse=True)
+        print('50 biggest memory tests:')
+        for t, name, mem in times[:50]:
+            print(f'{docname} : {t:.2f} sec., {mem:.2f} Mb')
 
     def _read_parallel(self, docnames, nproc):
         # type: (List[str], int) -> None