No I've gone into the user interface (in-game, as you have) and there's nothing that can be changed. The dev should look at the game Peasant's Quest (tho this would be a huge pain to do) in that game you can shut the lighting effects off.
The other thing that kills memory are the effects, leaves blowing past the screen, shadows, ect. My computer is also a potato despite it being fairly new, I think it was set-up for simple use, however thats not what I use it for. I've even used laptops that were suppose to be fast, but they also ran like a potato when it came to games. I don't even want to try my tablet or phone for the same reason.
The other way to eliminate the issue would take a long time, and needs to be done manually with an art editor. And picking the right files, which I'm guessing are packaged, and not un-packed. I have done this with another game called Claire's Quest, the devs have leaves, sand, cloud effects but I edited those effects with an art program and it makes the game faster. But the files are un-packed and easy to access. And I should point out with Struggle With Sin I haven't really explored the files I just play the game as it is, and in doing so I also contemplate throwing my computer through a wall or out the window.