My old kindle runs it playable with some system tweaking. It is loads older than the current generation of phones. I haven't tried on say a note 4 or s5 yet though. Like has been said again and again even though every single phone currently out has more raw mhz and many with more ram than a ps2 it is the architecture that is killing it. I have said screw emulating the hardware create a program to replace the emotion engine. Sony's emotion engine is part hardware but mostly software wrote to take advantage of the hardware. Now if you replace the emotion engine with something else that does all it does but is made to work with android hardware. Android and current hardware is more than powerful enough to render in real time as many polys at the same fps as a ps2.
Well the problem is no one is looking at it from the sand point of making a new engine that runs ps2 games and are instead looking at it from a emulating the hardware standpoint. As then it just needs to load the "bios" which is the engine software that makes a ps2 a ps2. Part of the reason it has not been done yet is i suspect legal reasons. Meaning it is reverse engineering software. Where as if you emulate hardware that then loads a bios from your ow ps2 there is no legal issue least of all not for the creators of the emulator. It is why for example the ps1 and ps2 emulators do not include the bios and they state it is illegal to share the bios or to download one.