What model touch do you have?
1. Depends on how you do it and what firmware you are running.
2. Once again depends what you do with your jailbreak, if you load your device up with all the bells and whistles using heaps mobile substrate addons, tweaks and bulk theming you may well experience slow downs and crashes.
3.Again depends on what you do with your jailbreak but in most cases it should not make much difference.
4.Never had any problems myself.