Will jailbreaking make my device slow forever?

I want to jailbreak my iphone 4 its a bit slow already but i wanted to know if it would slow my device forever as if i wannt to restore i want it fast again my friends have told me it will stay slow but i don't know someone tell me?

Iphone 4 error 21 stuck in recovery mode

My phone Iphone 4 is having a few problems. It won't power up without being connected to the mains or connected via a computer. When it is connected it comes up a little symbol saying connect to Itunes, and Itunes says that it needs a system restore, When I try this it comes up error 21 and refuses to reinstall the system. Here is the things I have tried so far.

* Tried it on a difference pc
* Tried reinstalling itunes and I have the updated version
* I have tried renaming the file in drivers/etc didn't work
* I have tried holding it down and the screen goes black then the apple logo comes
back up and nothing happens.

Here is my 2 questions, is there anything else I haven't tried that can save my phone?
Is there anything I can do to fix this

And i have made an appointment with Apple Store to try and fix it, is there any chance that Apple could save my phone?

How to Delete Iphone data Except Apps?

I'm planning to give my phone away to my friend so I want to delete all of my data off of it except for some paid apps that he wants. How do I delete all of my data except for apps and remove my Itunes account?