I am hearing from many people that after jailbreaking an ipod/iphone or having it jailbroke for them the most complaints I hear about is speed of the ipod from the safari, to even listening to music. All of this depends on what apps you run with the jailbreak, how you download your music( illegaly, or through iTunes), and your current storage.

Some things I have found is there is a lot more bugs in Cydia, so I pefer rock.

Installous when downloaded is a major factor there is also the data sharing which can be dangerous most of the time.

And illegaly downloaded music is often corroupt so I would reccommmended and conveter of some sort.

And my biggest advice is to update and re jailbreak your device to optimize you speed.