Various optimizations (#2096)

* Add roots to preprocessing route all at once

* Optimize Card header updates

panel/layout/card.py

@@ -57,6 +57,7 @@ def __init__(self, *objects, **params):
         self._header_layout = Row(css_classes=['card-header-row'],
                                   sizing_mode='stretch_width')
         super().__init__(*objects, **params)
+        self._header = None
         self.param.watch(self._update_header, ['title', 'header', 'title_css_classes'])
         self._update_header()
 
@@ -76,12 +77,20 @@ def _process_param_change(self, params):
     def _update_header(self, *events):
         from ..pane import HTML, panel
         if self.header is None:
-            item = HTML('%s' % (self.title or "​"),
-                        css_classes=self.title_css_classes,
-                        sizing_mode='stretch_width',
-                        margin=(2, 5))
+            params = {
+                'object': '%s' % (self.title or "​"),
+                'css_classes': self.title_css_classes
+            }
+            if self._header is not None:
+                self._header.param.set_param(**params)
+                return
+            else:
+                self._header = item = HTML(
+                    sizing_mode='stretch_width', margin=(2, 5), **params
+                )
         else:
             item = panel(self.header)
+            self._header = None
         self._header_layout[:] = [item]
 
     def _get_objects(self, model, old_objects, doc, root, comm=None):

panel/template/base.py

@@ -148,6 +148,7 @@ def _init_doc(self, doc=None, comm=None, title=None, notebook=False, location=Tr
         preprocess_root = col.get_root(doc, comm)
         col._hooks.append(self._apply_hooks)
         ref = preprocess_root.ref['id']
+        objs = []
 
         for name, (obj, tags) in self._render_items.items():
             if self._apply_hooks not in obj._hooks:
@@ -163,17 +164,18 @@ def _init_doc(self, doc=None, comm=None, title=None, notebook=False, location=Tr
                 sub._models[ref] = submodel
                 if isinstance(sub, HoloViews) and mref in sub._plots:
                     sub._plots[ref] = sub._plots.get(mref)
-            col.objects.append(obj)
             obj._documents[doc] = model
             model.name = name
             model.tags = tags
             self._apply_root(name, model, tags)
             add_to_doc(model, doc, hold=bool(comm))
+            objs.append(obj)
 
         # Here we ensure that the preprocessor is run across all roots
         # and set up session cleanup hooks for the fake root.
         state._fake_roots.append(ref)
         state._views[ref] = (col, preprocess_root, doc, comm)
+        col.objects = objs
         col._preprocess(preprocess_root)
         col._documents[doc] = preprocess_root
         doc.on_session_destroyed(col._server_destroy)