Also, there seems to be a competition for the keyboard, the game doesn't have full focus. When I press up/down, the page scrolls, despite mouselock. (Pretty scary, watching comments, no mouse^^)
Here's what I do to get forced, exclusive focus, despite itch's messing with the keys handler. I add to the canvas an onclick handler, that calls a function that will:
var mycanvas=document.getElementById('yourcanvas');
mycanvas.setAttribute('tabindex','0');
mycanvas.focus();
It could be, that by not having full focus, the thread has a low cpu priority.
Also, have a look at the js error console, there's a bunch of warnings.