For the smaller sizes it's fast enough, but for larger sizes I'd recommend showing completed chunks of steps at a time, instead of each individual diamond/square step at a time.
And the algorithm is "embarrassingly parallel" so you can run it really efficiently on the GPU if you want to make some huge maps.
It's also worth noting that the algorithm's speed here is tied to (I believe?) your monitor's refresh rate, since it's using requestAnimationFrame to loop without blocking the main thread entirely. I'd briefly considered a "complete" button which would run the whole thing in one go and render once, and that would enable reviewing for performance optimisation without hanging the page. But... I got too excited and wanted to share it first.