I'm afraid there's not much you can do at the moment - the game is not very well optimized but we'll be working on improving the performance in future updates. If the graphics are a bottleneck on your system you might be able to get some extra performance by disabling lighting and the line of sight -effect by entering "lighting" and "los" in the debug console (which can be opened by pressing F3). Unfortunately this won't work in the multiplayer though.
I'll close this topic as it's not related to modding: the
github issue tracker would be a more appropriate place for this.