Jailbreaking alone does not reduce the performance of your iPhone. However, you can end up with an iPhone that runs slower... for one simple reason. Once your iPhone is JB, you may be tempted to start installing all kinds of apps and/or background processes.
All of these installations, while adding functionality to your iPhone, will inevitably tax the resources of your iPhone. It's much the same way as how a home computer will start to run slower and slower as time goes on, and you install more and more stuff on it.
Worse case scenario, you can always restore and start over with a fresh JB. For the most part, you only lose SMS histories, icon placement and some data that's stored inside various apps, such as a logs and block lists from iBlacklist.
However, once you learn more about JB iPhones, you'll be more adept at finding the various database files and backing them up before you restore and JB again.
That is if you ever even feel the need to do a restore.