I dont even know what I can suggest you to do with that. Some games has preferences like different "engines" (idk how to describe it properly), or function that drops fps in order to save power (on Android at least), but I believe there is no such options in Eternum.
Honestly, I cant imagine why its lagging for you in such way.
It'll sound a little lame and funny, but have you try to reinstall/redownload the game?
![KEK :KEK: :KEK:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
Btw, maybe this is because of Beta version, although you're first who've discovered problem with it.