ZUBB is pretty processor intensive. Every time the index_html is hit, it does a look up through all the posts, finds the newest ones and for each one of those (depending on your batch_size) goes through them and counts the replies.

It is the counting of replies that is so intensive and where you should focus your optimization efforts.

• Reduce the batch_size and you will reduce the amount of topics it needs to fun through
• Remove the "replies" column from the "index_html"
• Use caching in the main page