Comparing Opera to IE is apples and oranges. I run mozilla under windows and Linux, and under both OSes it takes about the same to load. I'm sure running Opera under XP will produce the same type of slowdown that you will see under Linux, and thus the problem lies with Opera, or Mozilla in my case, and not with Linux or XP.
One thing right off that bat that you can turn off is sendmail. I found that with a lot of distros sendmail automatically starts up, and that can be a real hog. Type ps -a | less at a command prompt to get a listing of every process that is running. For a standalone PC I would turn off unnecessary networking processes, multipule getty processes, and gpm for starters.