I use Figma every single day for work, and have virtually no performance issues, even with projects with 100s of frames.

I just tried using the marker in Figjam, and it works perfectly fine for me. I think you may be running into an incompatibility between your browser or settings and Figma.

As for the infinite canvas, this is very handy as a designer, it makes it much easier to work across multiple frames at once, such as quickly zooming out to look at the flow or building a prototype, all the while zooming in to fix issues.