Improve performance when scrolling with many tabs.

Closes #1278.

CHANGELOG.asciidoc

@@ -33,6 +33,7 @@ Changed
 - Pasting multiple lines via `:paste` now opens each line in a new tab.
 - `:navigate increment/decrement` now preserves leading zeroes in URLs.
 - `general -> editor` can now also handle `{}` inside another argument (e.g. to open `vim` via `termite`)
+- Improved performance when scrolling with many tabs open.
 
 Fixed
 ~~~~~

qutebrowser/mainwindow/tabbedbrowser.py

@@ -613,7 +613,7 @@ class TabbedBrowser(tabwidget.TabWidget):
     def on_scroll_pos_changed(self):
         """Update tab and window title when scroll position changed."""
         self.update_window_title()
-        self.update_tab_titles()
+        self.update_tab_title(self.currentIndex())
 
     def resizeEvent(self, e):
         """Extend resizeEvent of QWidget to emit a resized signal afterwards.