it probably does too much on the CPU
Back in the previous decade GPUs had to transfer data back and forth using the CPU. It was sort of like the GPU was just a tool that the CPU got to use to do computation, but later on they improved the onboard memory for GPUs and allowed programs to shift to a new method of batching up everything and sending it all at once and forget about it. This shift meant that the GPU was independent of the CPU in most respects allowing higher frame rates.
So the issue is that the game was programmed to work on old Japanese graphics cards. It was also probably entirely single threaded meaning there's no chance you'll ever get it to play fast.