Don't quite get where you are coming from as you seem to misunderstand a few things.
First and foremost, nothing online (apart from maybe using file sharing or rogue sites with constant pop ups) is going to slow down your PC. You say these mails are in your bulk account (spam) and these are NOT downloaded when you use outlook, so they have no bearing on anything. They are also deleted periodically, tho new ones take their place. It's spam, you can't stop it. On the same subject, if I were you I'd actually check this folder online a bit more often - mine picks up spam but it's also been known to pick up stuff I wanted.
I would suggest something else is wrong - a program running in the background that can be got rid of maybe. Try running a virus check too, or try Defragmenting the drive or cleaning with window washer or C Cleaner.