What is the fastest iOS version for the iPhone 3GS?
Ok, so after some quick googling and reading some Apple forums, it looks like iCloud (in iOS 5.x) isn't ready for prime time. I've done the following and now iOS 5.x seems a lot snappier. 


Disabled iCloud everywhere (Settings -> iCloud)

Disable all apps then Scroll down and hit "Storage and Backup" and disable iCloud backups. If you really want to go strong, just delete your iCloud account setting (on your phone, Settings -> iCloud -> scroll to bottom)
Disabled sending data to Apple (Settings -> General -> About -> Diagnostic &amp; Usage -> "Don't send")
Minimized Location Services (Settings -> Location services)

I kept it ON globally, but disabled it for certain Apps
Most importantly, disabled it for "Diagnostic &amp; Usage" as well as "Location based iAds" (Settings -> Location services -> Scroll to bottom -> System Services)
Disabled notifications for the calendar (Settings -> Notification)

I also removed notifications for many other apps that had added themselves ...
However, I suspect this is fixed in iOS 5.0.1 under their "Battery issue fixed" patch. Earlier, Calendar notifications would hang causing CPU to be stuck 100%, draining the battery and leaving little CPU for other items.


Also, there are two apps, SysStatsMon and System Status. Both are good tools to see CPU usage on your iOS device. About 5+ seconds after launching them under normal circumstances, your CPU should be in the 5-15% range.