[horde] how do I get cache to work

[Oliver Schönrock]

I am running horde 2.1 with IMP 3.1, turba 1.0, kronolith 1.0, nag 1.0, and 
mnemo1.0.

I have configured cache in horde.php for file system storage as follows:

/**
 ** Cache System Settings
 **/

// If you want to enable the Horde Cache, select a driver here.
// This is used to speed up portions of Horde by storing
// commonly processed objects to disk.
// Valid values are 'none' (don't cache any objects),
//                  'file' (store objects in filesystem)
// $conf['cache']['driver'] = 'none';
$conf['cache']['driver'] = 'file';

// Any parameters that the caching driver needs.
// $conf['cache']['params'] = array();
$conf['cache']['params']['dir'] = 
'/home/sites/www.schonrocks.com/web/horde/tmp';

but I have been monitoring the above directory for any cache files, but so 
far I can't get any to appear during normal use of horde applications.

My goal is to speed up the script running times, particularly for imp. 
Currently it takes 5 seconds (with insignificant data transfer time 
included) to pull up the inbox for the first time (and it doesn't improve 
on subsequent requests in the same session).  Will cache speed this up? 
(obviously there are many delays involved here including the imap delay, 
but I thought cache of the objects should have some benefit).

How do I know it is working? If it is not working (i presume it isn't) then 
how do I get it working?

Thanks is advance

Oliver Schönrock