When I first played the game it was very slow. Is that still the case?
This game is made with RPGMaker, and RPGMaker uses Chromium to display things. If the game feels painfully slow, Chromium is probably not using your GPU to render the graphics and is loading up your CPU instead. There is an option you can pass to Chromium that can force it to use your GPU, and this speeds things up immensely. Here's what worked for me on Ubuntu, it should work similarily on other platforms.
In the main folder of the game, there is a file named "package.json". Open it in your favourite text editor and look for a line that says "chromium-args". If there isn't one, just make one on a new line somewhere between "name" and "window" and it should look exactly like this:
"chromium-args": "--ignore-gpu-blacklist",
Don't forget that last comma. If there is already a chromium-args line, add "--ignore-gpu-blacklist" inside the quotes after any options that are already there. Make sure there's a single space between what's already there and what you are adding. Mine already had an option and now it looks like this:
"chromium-args": "--disable-setuid-sandbox --ignore-gpu-blacklist",
All options inside a single pair of quotes.