Comment by tomlagier

Nit: Foundry uses Pixi.js for rendering, so it's using more or less standard JS gamedev tools to draw on a WebGL context in a canvas.

Edit: See https://foundryvtt.wiki/en/development/guides/pixi

Edit2: For an example of why gamedev toolkits don't necessarily produce performant, highly usable software, check out Dungeondraft (https://dungeondraft.net/). It's built with Godot and gets the job done, but as an application it's a total mess. I'm working on an alternative but (surprise!) it's a challenge.