These files are not supposed to be emptied, and you should not empty them unless you are sure.
The cleanup creates these compressed files in order to save space. They are files you have not used in a long time, since you rarely use them, cleanup compresses them.
When you do go to access these files in the future, they will be accesses slightly more slowly the first time (since it needs to decompress the files). Cleanup compresses these files, to save space on the hard-drive.
The slowdown is not due to these files. There is likely some startup issue like the other poster suggests. lately is is often due to people installing 2 antivirus software packages. If you have 2 installed, remove both then install 1.
Use MSConfig to find the cause see http://www.capitaltechmd.com/index.php?option=com_agora&task=viewtopic&pid=95&Itemid=19#p95
Astrophel, February 2009