Why is phpBB 1.x faster than phpBB 2.0?
The original phpBB 1.0 saw a very large decrease in page generation times from version 1.0/1.2 to 1.4. This was achieved by rewriting many of the SQL queries and splitting the database. These changes came about from work on phpBB 2.0. With phpBB 2.0 we have introduced numerous additional functions and features, not least of which is full templating (the separation of page design from code). This results in a slowdown from phpBB 1.4, we suspect though that the new features will outweigh the small difference in page generation times.