It's written in assembly language! And the web player is due to a really cool feature of this one online C64 emulator: https://lvllvl.com/c64
Oh that's quite impressive! I just started C64 Basic last year. Assembly is a bit too difficult for me. For now : p
But using mospeed gave me a pretty good result (in speed) anyway : )
Thanks for the link! I will check it out. It's easier to make people play my game. Then then they don't need to download it and stuff.