for what it's worth, finally I decided I had enough of this EPG stuff and switched to OHS, performance is back to normal (with or without squid proxy server).
These steps are what I did:
1. make sure compat-db RPM is installed. I have a CentOS X64 OS 5, compat-db installation is not enough, had to go /usr/lib to make a symbolic Link:
# ln -s ../../lib/libdb-4.3.so libdb-3.3.so
( otherwise OPMN can't start ....)
2. download 32 bit Linux OHS software from 11G download ( there is NO 64bit Linux version).
3. Run installation, which is straightforward.NO options available for choice. NO password asked...
after installation, only HTTP server works(good enough), Application Console not working(don't bother)...
Looking back, I wish I never know things like EPG. The OHS is better , for performance, for documentation( looked into DBMS_EPG, DBMS_XDB but details are hard to find ) and for management( just the httpd access log is more normal....)..