hey hey! I have an iphone 4 (Outdated I know) I have had it jail broken since the day i got it with no real issues. I do find over time things will be updated and the phone and apps will have a tendency to slow down or crash from time to time. Not so much as to be an annoyance though. The best part is you can back up your iphone thru itunes and always revert back to the backed up settings. I just put iOS 6 on mine and am yet to perform the new jailbreak.
Did. If you have SBSettings, press Processes and then Free Memory. It should speed things up a little. I do it from time to time when I feel things get sluggish.