Mine isn't any slower now its been jail broken.
I mainly did it so I could have my home screens how I want them not Apple, so I can have however many apps in the dock, I can scroll the dock, I can resize icons, have a grid of 6 by 6 icons on screen etc. At present I have no icons on my first home screen, you scroll up the screen to bring the apps up from off screen. Of course I have other useful jb programs like sb settings and my fav which removes the lock screen.
Apple won't let me do customise anything but the wallpaper which Im nit willing to put up with, if you couldn't jb then I would go with Android.