I also would recommend to use APC, installation takes only a few minutes, and further optimization of the configuration is also not a very time consuming task
On the other side:
What are the specs of the server FengOffice is running on?
Which OS, PHP & MySQL Version, wich Web Server. And, also the configuration files, memory limit of PHP, the limits of MySQL, is the Fengoffice Database in a InnoDB enviroment or MyISAM, etc.