Improve the performance of MCP

At present it takes 5162 ms to run the database queries on that page (MCP Home) for a site the size of .com (More than 95% of request time).

The problematic slow queries are those called in these methods:
phpbb\notification\type\base::check_user_notification_options (2380 ms)
delete_topics (1750 ms)
sync (670 ms)
update_posted_info (330 ms).
The rest are under 10ms