> Maybe the canvas operations are particularly slow on Firefox

That seems likely. WebRender (Firefox's GPU accellerated rendering backend) doesn't do vector rasterization. So Firefox rasterizes vectors using the CPU-only version of Skia and then uploads them to the GPU as textures. Apparently the upload process is often the bottleneck.

In contrast, Chrome uses (GPU-accelerated) Skia for everything. And Skia can render vector graphics directly into GPU memory (at least part of the rasterization pipeline is GPU accelerated). I would expect this to be quite a bit faster under load.

It's a known problem, but I hear that almost all of the Gecko graphics team's capacity beyond general maintenance is going towards implementing WebGPU.

---

SpiderMonkey is also now just quite a bit slower than V8 which may contribute.