Does jail breaking your iPhone 5S slow it down bad?

I know it slows it down by jailbreaking but will it make it so slow or just a little slow? I want to jailbreak my iPhone 5S and what's the worst that could happen while jailbreaking?

Worst case scenario? They take over your phone, sell your information and ask for a few hundred dollars to unlock it. After that, they tell you to pay an additional sum and so it goes on.