Zend Optimizer and Xcache make phpBB 3 faster

I moved one phpBB forum to a VPS. All fresh.
I have Apache installed and MySQL. phpBB 3 relay on them.
Because of the size of forum, about 500K posts, 1000 users, the forum is not as fast as I think.
I have to do something to improve the performance of it. I still remember that some opcode optimizer can help.

OK. I did install Zend Optimizer and Xcache.
The version of them are as below:
Apache 2
PHP 5.2.10
Zend Optimizer 3.3.9
Xcache 1.3.1
I give 96MB to xcache, 16MB to Var.
The average load time is reduced from 0.198s to 0.082s
It is a huge improvement.
Actually I tried memcache before xcache. memcache can reduce about 25%, or 0.143s.
But, it is clear xcache is a way faster on phpBB 3