The only other web app I can think of doing the same thing is Google Docs, which uses it's own canvas based renderer with an svg overlay for accessibility. I feel it's telling that Google thought leaning on their own browser (through HTML/CSS) wasn't even good enough for typesetting.
https://workspaceupdates.googleblog.com/2021/05/Google-Docs-...
On Linux + Firefox, basic navigation, like panning and zooming around in a file, is painfully laggy. In FigJam, try using the marker to draw any curved line--it updates at like 2 frames per second, which makes it completely unusable for handwriting text or drawing diagrams. Excalidraw, on the other hand, in the same browser, is perfectly lag-free.
Every time a designer sends me a Figma I just ask for exported images because using the website is an exercise in frustration. And don't even get me started on the impossible, spaghetti-on-an-infinite-canvas UX requiring endless zooming and dragging...
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.
I also happen to use Linux + Firefox and so I have to agree that it is definitely not a joy to use Figma, even for small projects.