[TigerC10]

You won't be able to use HTML, the PHP is used to render dynamic content.

Have you checked the vB error logs? Sometimes they can clue you into some problem that is occurring frequently which slows down the site.

Also, have you optimized the database at all? The more activity on a board, the more overhead gets generated for the database tables - the more overhead, the longer it takes to query from a table. I try to optimize the tables at least once per week.

Finally, large user databases can slow down a lot of the CRON jobs that run every hour. Run a user search in your AdminCP and delete all of the people who have 0 posts, and it should clear things up.