The jailbreak itself does noting to adversely effect the performance of the device. It simply unlocks the operating system.
What you install on the device AFTER you jailbreak, in the way of tweaks and apps to enhance the OS will most certainly effect things. Just like your computer, the more things you set running in the background doing things, the less resources there are to work with.
chantelle310 is also correct, the biggest culprits for very poor performance are where you load an app or tweak which was designed either for another device, another version of IOS, or which clashes in functionality with another tweak you already installed.