The problem with the Rasperry pi is that it needs to start up the operating system, like Windows, or a smartphone. You don't want to wait for starting up your viewer to see a short show ;-)
If you are using the classical Raspbian, the boot time can be "long", but customized system aimed toward a specific application can be faster by cutting unnecessary features.
In this article <https://www.furkantokac.com/rpi3-fast-boot-less-than-2-seconds/> the authors have a 3s boot (including display). As often, the last milliseconds are the most difficult to gain, but they say that without effort (using official buildroot project) they started with a 10s boot [which does not surprise me]. Thus it should not be overly hard to have something in the 5s range [given in addition that they provide the modified sources] with a static picture that can be displayed earlier to get shorter 'subjective' boot time.
Of course, this does not work if you want a generic system.
So most likely, we would have a platform that is compatible with Android.
I am not saying that Andriod is better than Raspbian, but compatibility and the ability to exchange data on an easy (known) way is also worth something.
I lost the reason why you are talking about this, but from the software point of view it is essentially a choice between fast and specific/rigid [custom system] versus slow boot but versatile.
Programming an application for one platform or the other is about the same difficulty. It is even more true if you code like I did for PhereoRoll3D since this app compiles for the two without changing the code.
If you do special hardware, I would advise to use a custom system because porting Android was a quite horrible (and thus expansive) experience.