Downloaded the latest version last week, been playing in 30-60 mins spurts the past few days and no lag issue on my end - Have you tried looking at your Task Manager to identify bottleneck? Or maybe are you using your imbedded CPU's GPU which are usually not great at rendering 3D animations (e.g. Intel HD Graphic 500)
On a Win10 PC, go to Settings > System > Display > (all the way down) Graphics settings, then click Browse and select the Eternum.exe, and finally select "High performance". This window will also tell you if you have a base and a high card, if you have no choice then that's it.
Sorry if this does not solve the issue - hopefully v0.6 will help you or write to Caribdis with the version you are using and a brief summary of you settup (OS and OS version, graphic card and current driver version, which .exe you are using (Eternum vs Eternum-32)) and they might be of better help