It's fairly weak, but might be good enough considering the zenfone 5 is 720p. Higher resolutions are pretty, but they take too much RAM and GPU power to run. 720p seems to currently be the sweet spot for phone gaming, and the PowerVR SGX 544mp2 should keep playable framerates in the majority of games, but it's probably a good idea to check YouTube videos for gameplay just in case.
I don't know exactly how heavy ZenUI is, but when I was considering buying it myself, reviewers typically thought it was pretty light and smooth.