Depending on the machine you try (or your DOSBox settings), you might get equal or much better performance to the GBA. On my 486SX 33MHz, the game is silky smooth (or at least as much as the passive matrix LCD allows me to see). The GBA is much weaker than that, but there's also the point of the level design not being totally optimized yet. I'm working on a proper level editor to help me design efficient levels by default.
As for what to do - this demo was much more a testing of waters than really a proper game. I wanted to know if people would find the combat weird or maybe the graphics too grainy or ugly. I was also trying to flesh out the atmosphere I wanted and once I finally got to that point, I can organize my asset pipeline and tools around that.
This is just the grain of sand that will, hopefully someday, create a good RPG :)
Thank you for trying and stay tuned for new developments!