Yea I had jail broken my phone several times. I also had to restore it back several times. After a while the phone becomes very slow and unstable. What makes it worse I wasn't even using a theme. I was just using no more then 5 tweaks. Your best bet is to just keep it stocked. If you are going to jb just becareful of the tweaks you download.
I have over 30 tweaks installed rite now and my phone is just as fast as before I jailbroke.
However everyone and a while there is a tweak that really bogs down your device by itself.
One that comes to mind is Lockdown Pro. It's a great tweak...and stable within itself....but it will make your phone lag on its own. Shame...good tweak...it's just too heavy I guess...and causes what I would consider a significant slow down.
I removed Lock Down Pro and the lag went away.
AppLocker works better.
Another tweak that can *slightly* slow down your phone is Music Controls Pro.
It will slow your phone down ever so slightly.....but considering what the tweak does its totally worth it...since I listen to music as much as possible.
The best thing to do with MC Pro is turn "Task Controls" off..and use your own Activator/QuickDo gesture...and make sure the nonfiction center addons are off as well....which they are by default anyway.
But in general speaking...it takes about 35-45 tweaks installed before the phone starts to slow down(provided the tweaks you have are compatible, work well, and don't conflict with others).