: I don't have time to write a very long response to this, I'm afraid, but I have had very good experiences using MyBB, which is essentially just like vBulletin, but free and open-source. It

I don't have time to write a very long response to this, I'm afraid, but I have had very good experiences using MyBB, which is essentially just like vBulletin, but free and open-source. It has timely security fixes, has a useful, if not a very large, community and the software is in my experience fast, easily customizable, secure and has great features with some even better plugins.

I do not have experience using it with such a large forum, however, so I can't tell you it will be a perfect fix for your situation, but it does have a simple import/migration tool built-in, which is more than I can say for some of the payware alternatives.

It is PHP/MySQL based.

It is cached well in my experience, although you can further increase that using server-side caching if necessary.

www.mybb.com/features/performance/
Built-in support for eAccelerator,
memcached and X-Cache caching.

In a standard installation of MyBB, we
already cache the most frequently
accessed items (a list of forums, user
groups, birthdays etc) to either the
database or the file system. While
this is a great speed and optimization
improvement, having this information
stored in memory on the server is
quicker once again.

MyBB also has the ability to store
cached information using built-in
shared memory caching from PHP modules
such as X-Cache and eAccelerator and
also has the ability to interface with
memcached servers for cache storage.

www.mybb.com/

Good documentation:
wiki.mybb.com/index.php/Main_Page