Jailbreaking does not slow down your device. When you jailbreak and the device starts to be slow, it's because the jailbreak is on top of the core OS and you need to restore your iPod to rectify the problem. A large number of application ones you jailbreak your device also contribute to your device being slow.
Yes, it does. My friend jailbroke his iPod Touch, and its been slow ever since. If you jailbreak your iPad and this happens, you can restore it by going to iTunes and changing it back. It will restore to its original speed.