Hi! I have a three month old macbook pro. It's hard drive randomly started to fail (click of death) after three weeks, with no apparent cause such as accidental damage. I brought it in to get fixed and the hard drive was replaced, so I restored from time machine using an external hard drive. Everything is back in place, but the computer seems to be running a lot slower than before the hard drive replacement. It will randomly slow down, and it's definitely not the internet connection, but I do notice that when I initially start up, no matter how many programs I am running, it is fine.
Try using a program called OnyX, this will sort out any preference files or applicatioms that may be slowing it down. If no luck try creating a new user account and copying all your files over to the new account and deleting the old one, worked for me.