Hello,
Here are some of the suggestions that I can give you :
1. Run top and see which process is consuming the most CPU and Memory.
2. As far as I see from your Apache config file, you are using Apache prefork mode. You can try installing(compiling or with apt-get) Apache with worker mode and fastCGI for PHP. I believe on older PCs, websites run faster on Apache with worker mode and fastCGI.
Here is an article how to install Apache with worker mode and fastCGI from apt-get :
3. Try using memcached for your server. ( You can find a lot of information about that on Google )
Here are some example articles for installing memcached for PHP, MySQL and the memcached daemon :
This will help you install memcached daemon and set it up to work with MySQL.
Here you can find how to install memcached extension for PHP and how to compile the memcached daemon from source code.
I hope this helps