Lemmy

Nah, I use Firefox and lemmy is fine for me. It's also fine for pretty much everything else I use it for, and I use it on my phone (Android), work laptop (macOS), and personal computers (Linux). I haven't used Windows in over a decade, so I can't say much about that, so maybe there's an issue with Firefox on Windows. Also, it's funny that you blanket blame open source software for being slow in the same breath as blaming Windows, and I'm guessing you'd say that Linux is well optimized.

And Jerboa is also an open source project build by one of the two core developers that work on lemmy, and most of the patches come from the community. The same is true for lemmy's web UI.

So I'm not really sure where you're getting the idea that open source projects tend to be poorly optimized compared to proprietary projects (also, Chrome is based on an open source project, Chromium). Projects can be well optimized or poorly optimized, and that's independent of the licensing of the source code.

If lemmy's UI is slow, it's because it's an immature project still in an alpha stage of development. Or maybe OP is on an unstable instance and a bunch of requests are failing, or maybe they tend to browse image-heavy communities and thumbnails are poorly optimized or something. But it's unfair to blame any apparent performance issue on the licensing of its source code.