nobody is cloning your mac address. Your mac address is used to identify yourself to the ubr and it is unique. If someone had cloned your mac and was using it then you wouldnt notice your pc slow down, your pc would be fine but you just wouldnt be able to connect to the internet because someone else is already connected using your info. The only time you might notice a degradation in peformance on your pc is if you had a virus or something else getting up to mischief and your pc was being used as part of a botnet or something, buuuuuuuuuuuuuuuuuut, your security suite would show this up. If you have got av software installed, update to date and it is saying everything is cool then dont worry. If your pc is slowed down it will be because of the av software doing realtime monitoring and stuff like that. Things will be going on in the background which you cant see. Norton (one of the more popular) is one of the worst for impact on system peformance.
Take a chill pill dude
