In a word: Nope!
To me, a jail break ENHANCES the phone, not inhibits it.
I do more with my jailbroken phone than can be done with a stock phone. Yet, it still works just fine.
All jailbreaking does is allow you access to the phone's operating system. Then, you can install tweaks that will make the phone run better ... for you. I have about 5 tweaks I would sorely miss if I had to go back to stock.
If you stay away from the dodgy repositories/tweaks (that mainly exist to help people steal apps/tweaks), your phone will be fine.
Marilyn