I have some Drupal sites and the one common problem on them: very large cache with very slow processing and therefore the sites work very slow. I have enabled all caches and set them to long lifetime. But the sites are still slow. My server has 4GHz frequency and 4G RAM. DB queries log shows me that all queries takes 100-300ms summary.
When I perform profiling with xhprof I see that the largest part of wall time is used by serialize, unserialize and array_map functions. So i suppose it is because of large amount of serialized data stored in cache. So the data can be get from DB quickly but is processed by CPU very slowly.
So what should I do with the issue? How can I reduce cached data amout or store not serialized data in cache?