I'm no expert in jailbreaking, but doesn't it slow down your device?
Just asking 'cause I've an iPhone 3GS.
nope
It can do. Jailbreaking in and of itself doesn't do much to slow down your phone. However, it enables you to install system modifications of your choice which can severely slow down an iphone. Especially if you have many of them all running at the same time.
For me with my iphone 3G, it was worth it to make iOS 3 minimally usable (instant access to brightness, sms, and airplane mode) without slowing it down with iOS 4.