First off I am using a android tablet not a phone, so my experience may be slightly different.
Disconnect from server, happens constantly at least 3-5 times a day, regardless of how close I am to the router. Device is set to keepalive connection.
Ads not loading, causing game to lockup and crash.
Minor sound lag now and then, with it coming a few seconds after the weapon fires.
Boss fights having extra lag to them without warning, been disconnected in the middle of a boss fight several times?
So yeah, other then that I do like this game so much. Woo.