Collect some timing stats on deserializing data files (#6507)

Author: msullivan

mypy/build.py

@@ -1777,9 +1777,17 @@ def load_fine_grained_deps(self) -> Dict[str, Set[str]]:
     def load_tree(self, temporary: bool = False) -> None:
         assert self.meta is not None, "Internal error: this method must be called only" \
                                       " for cached modules"
-        data = json.loads(self.manager.metastore.read(self.meta.data_json))
+        t0 = time.time()
+        raw = self.manager.metastore.read(self.meta.data_json)
+        t1 = time.time()
+        data = json.loads(raw)
+        t2 = time.time()
         # TODO: Assert data file wasn't changed.
         self.tree = MypyFile.deserialize(data)
+        t3 = time.time()
+        self.manager.add_stats(data_read_time=t1 - t0,
+                               data_json_load_time=t2 - t1,
+                               deserialize_time=t3 - t2)
         if not temporary:
             self.manager.modules[self.id] = self.tree
             self.manager.add_stats(fresh_trees=1)