PHP:
$config['cache']['backend'] = 'Apc';
I'm not sure it's actually needed but before I was using 'File' and I've figured it would be better to use memory instead of disk.
Anyway, you can test without and if you see it's filled in User Cache Entries when checking apc.php ; I guess you don't need to specify backend.
I can't say if I really see a different, as I did not test really without before, but it should help fore sure considering when you look at apc.php it's well filled with entries and hit a lof of time.