Do I have the video for you:
tl;dr: you have to have the Godot source code to compile export template for yourself (it also allows you to encrypt your game, kinda useful) and you strip-off Godot's component your game doesn't use (spoiler: a lot)
All the tools are a bit annoying to install but the few hours to understand and do so are worth it
For the web you can also use brotli compression on top of that.