Are you claiming that browsers, their renderers, V8 etc. are slow and unoptimized?

My claim was much simpler, you've just ignored it as a junior who's never seen an app deteriorate over time because that's technically impossible, progress only marches forward!

But to your point - sure they are, not only in RAM, but also disk space, battery use, and startup speed etc. Some parts of them are fast, but in that suboptimal stuck-in-a-slow-niche kind of way like V8, which a better alternative would just get rid of (no doubt, great engineering once you learn about the almighty "trade-offs" that can excuse any performance failure)