Add a cache for rendered stylesheets

Otherwise, when showing hints a few times, we spend around 8-10s just in jinja
generating stylesheets.

qutebrowser/config/config.py

@@ -551,6 +551,15 @@ def set_register_stylesheet(obj, *, stylesheet=None, update=True):
     observer.register(update=update)
 
 
+@functools.lru_cache()
+def _render_stylesheet(stylesheet):
+    """Render the given stylesheet jinja template."""
+    # Imported here to avoid a Python 3.4 circular import
+    from qutebrowser.utils import jinja
+    template = jinja.environment.from_string(stylesheet)
+    return template.render(conf=val)
+
+
 class StyleSheetObserver(QObject):
 
     """Set the stylesheet on the given object and update it on changes.
@@ -574,10 +583,7 @@ class StyleSheetObserver(QObject):
         Return:
             The formatted template as string.
         """
-        # Imported here to avoid a Python 3.4 circular import
-        from qutebrowser.utils import jinja
-        template = jinja.environment.from_string(self._stylesheet)
-        return template.render(conf=val)
+        return _render_stylesheet(self._stylesheet)
 
     @pyqtSlot()
     def _update_stylesheet(self):
@@ -613,6 +619,7 @@ def init(parent=None):
     val = ConfigContainer(instance)
     key_instance = KeyConfig(instance)
 
+    instance.changed.connect(_render_stylesheet.cache_clear)
     configtypes.Font.monospace_fonts = val.fonts.monospace
 
     config_commands = ConfigCommands(instance, key_instance)

qutebrowser/misc/utilcmds.py

@@ -37,7 +37,7 @@ from PyQt5.QtWidgets import QApplication  # pylint: disable=unused-import
 from qutebrowser.browser import qutescheme
 from qutebrowser.utils import log, objreg, usertypes, message, debug, utils
 from qutebrowser.commands import cmdutils, runners, cmdexc
-from qutebrowser.config import configdata
+from qutebrowser.config import config, configdata
 from qutebrowser.misc import consolewidget
 
 
@@ -169,7 +169,9 @@ def debug_all_objects():
 def debug_cache_stats():
     """Print LRU cache stats."""
     prefix_info = configdata.is_valid_prefix.cache_info()
+    render_stylesheet_info = config._render_stylesheet.cache_info()
     log.misc.debug('is_valid_prefix: {}'.format(prefix_info))
+    log.misc.debug('_render_stylesheet: {}'.format(render_stylesheet_info))
 
 
 @cmdutils.register(debug=True)