I am guessing an iOS App's size has direct impact on its launch latency, since iOS needs to load the App into memory before launching it. If so, I wonder how much is the impact?
Let's say if I reduce my iOS app from 100M to 80M, how much launch latency improvement I can expect?
Does anyone has experience on this?
Consider that programming a faster App Launch experience is not bound to App Size at all. You could assume even the opposite because in some (i would even say in most) cases coding a faster launch process to start up can (and will) involve more code to make it a very short interaction interrupt experience and so your app size would increase without even delivering more valuable content.
In other words just because you deliver more dynamic or static content does not increase launch time unless you load and instanciate everything you need later on in your app life cycle in UIApplicationDelegate instead of allocating objects at the time of its use.
The same ideals apply to tear down processing (dealloc or saving app state user defaults) if needed. But this experience is usually not seen as a visible interruption of interaction with the device and so often forgotten to be part of the apps life cycle.
You can avoid this prolonged start up experience for users with clever use of LaunchScreen storyboards or similar techniques like partly buffered content for the first seconds. Some apps even take screenshots to be presented at the next start up to make the user feel the last app state is loaded already while building up the real interface.
A common mistake, which you may have seen also very often, is to make use of introduction videos or animations to bridge the time in waiting for the window/view to appear after app launch. So just making use of Launch Screens is never the best solution but a wise decision if the content that needs to be loaded first hand will take a noticeably longer time and can not be avoided.
LaunchScreens are just entertaining/informing about processing and keeping the user engaged while it can be beneficial what you and your app want to represent.
So abbreviated launch time is clearly a design pattern issue and is not bound to app size. Ok, apart from downloading band width if that is needed.