*FIRST OFF, I APOLOGIZE FOR THE WEIRD TWITCHING OF THE VIDEO! I don't know why Sony Vegas was doing this, and I wasn't going to render it again, but hopefully you can tell which one starts faster.*
A quick video comparing the app launching time of a jailbroken iPhone 4 (4.1) (with over a dozen MobileSubstrate extensions enabled), and of an iPhone 4 (4.1) without any enabled (aka. Safe Mode). You also might get some better performance if MobileSubstrate is just uninstalled.
While the differences aren't huge, it's definitely noticeable. If you open the same set of apps all the time, you'll see a noticeable difference.
Jailbreak at your own risk. Installing all the cool stuff can noticeably bring the overall speed and performance of your device down quite a bit. Jailbreaking itself doesn't change the performance, but MobileSubstrate does. Even by having it installed will slow things down.
I've uninstalled MobileSubstrate. The only things I have installed are PdaNet (for wifi/USB tethering) and iFile.
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