Immediately after I installed iOS4 on my iPhone 3G I reported that it seemed surprisingly fast. However, within two days, whatever pleasant surprise I felt about my iPhone 3G iOS performance came crashing down, and I do mean crashing.
The iPhone iBooks application, for example, crashes, a lot. Try to open a PDF in iBooks? Crash. Try to read a book? Crash. Time to wait for iBooks to open after you've downloaded 10 eBooks? I have no idea, I'm not that patient.
The Facebook app on my iPhone 3G w/ iOS 4? It crashes at least once out of every five startups.
Shoot, even the iPhone iPod app now crashes.
iPhone 3G iOS 4 - Performance problems, and more heat
The iPhone 3G iOS 4 networking performance also seems slower, though I haven't tried to document its performance issues. I know this will sound crazy, but since Apple "fixed" the "number of bars" problem, my iOS 4 iPhone 3G now shows only 1-2 bars here in Wasilla, Alaska, but, the crazy thing is that the performance also seems worse. As a programmer, I can't see the logic there, but I'd bet some money that the iOS4 iPhone 3G network performance is worse than the iPhone OS 3.x performance.
The iPhone 3G does also get very warm during phone calls, though to be fair I have been on some calls lately of thirty minutes or more. But, it does seem much warmer than before, not quite hot, but very warm.
If you're thinking that these iPhone 3G performance problems are due to the number of apps, contacts, photos, or other data I have installed, you'll be sadly mistaken. I've installed only three iPhone apps so far, and I messed up my contacts, so I only have one phone number in the system. I also only have four photos on the phone, so that sort of data is not an issue. I have over 1,000 songs on the phone, but that's also less than I used to have.
iOS 4 iPhone 3G Mail woes
The iPhone Mail app is also a bugger from time to time. Even here at home on my WiFi network it will just hang several times a day. And last night I noticed one of the really big iPhone 3G performance issues happens whenever I am receiving new email messages; the entire system grinds to a halt. You think the iPhone 3G is dead, and is going to need a reboot, until the you hear the "ping" sound, and the email is received, and iOS 4 suddenly starts responding again.
All in all, the iPhone 3G iOS 4 performance is, well, a bugger.
Update: August 18, 2010
Dear Apple: Please make my iPhone 3G iOS 4 applications quit crashing. :( The iPod, iBooks, Facebook, and many other iPhone 3G apps keep crashing so often now it's almost comical. Really, it's a sad feeling knowing that one out of every three apps you start is going to crash or be shutdown before you can use it. iBooks on the iPhone 3G with iOS 4 is utterly useless.
Where's the PC Guy these days? He can finally laugh at the Mac guy, or at least the iPhone Guy.
I had a lot of these same issues too. And all of the "fixes" I found do not really work because you're essentially just resetting the software. From my black-box programmer's perspective, it seems that the OS just doing too much in the background and is managing the 3G's limited memory poorly.
After some digging, I managed to dig up some 3.1.2 IPSW files, do a "restore" and use RecBoot (through Ubuntu) to get the old OS back on it.
No, it's not the proper solution, and I literally got a headache during the process (though that's probably coincidence). But it HAS kept me from smashing it with a hammer. :-) After adapting to the sluggishness of iOS4, the old OS is dreamily responsive.