Ubuntu is heavy for a Linux distro, because it uses the heaviest DE (GNOME), uses the less optimized Snap packages, and perhaps has other Canonical telemetry or something.

If you want better performance, try something with a lightweight DE. I have a laptop running Lubuntu (essentially Ubuntu with LXQt instead of GNOME), and it’s actually quite responsive, at least for basic system functions.

Because if you run anything on the web with a 10 year old CPU, it’s gonna suck due to the huge web browsers accompanying the bloated websites. Even on a well optimized website, the browser overhead is significant on bad hardware, especially regarding the launch time of the browser.