Speaking strictly from personal experience, after upgrading to Apple's latest iOS 4.0 and 4.0.1, my iPhone 3G, originally on iOS 3.1.3., was unusably slow. It was disappointing to see the iPhone 3G become something I hated to use. After looking up several tips and tricks to speed up the iPhone, including disabling all my Spotlight search results and double hard resetting the phone (which didn't seem to work very well), I decided to give jailbreaking another chance.It turns out that the jailbroken iOS 4.0.1 , even with multitasking and the background wallpaper service turned on, was noticeably faster than the official iOS 4.0.1 from Apple. I found this a bit strange. I instantly noticed it being slightly faster right after jailbreaking it. I still resorted to rebooting my iPhone a few times a day to keep it in working order. I used the jailbroken version with as full a feature-set my iPhone 3G can physically support for about a week, after which it begun feeling a little too sluggish for practical use.
Last night, I decided to re-jailbreak my iPhone 3G, this time disabling multitasking (because quite frankly, it was impractical on such a slow device) but keeping the background wallpaper settings (because I have grown used to having a non-black background). So far, so good, sort of. It definitely feels zippier than both iOS 4.0.1 by Apple and the jailbroken iOS 4.0.1 with mutitasking. Still, compared to the good and familiar 3.1.3, there is a no contest. (The only thing keeping me on iOS 4 are the folders and unified mailboxes.)
Supposedly, Apple's investigating this issue. I wait impatiently.
P.S. There's definitely some power issues that I've noticed with iOS 4, at least on iPhone 3G.