Just being jailbroken doesnt slow your phone down in itself. Just mywi? Maybe a little when activated but probably still not noticable. And if not activated then your phone should still run fast. On the 3gs, i was able to tell the difference in speed/battery just when i was using winterboard. Now i have that plus many more tweaks activated on my ip4 and i still dont notice a decrease in speed. Battery yes but speed decrease no.