By definition something that executes JS and parses CSS is a browser.
and yes it’s lighter than windows, proved by ubuntu being recomended for lightweight OS
Absolutely not. It gets recommended as a lightweight OS because 1) there are delusional people and 2) if you remove and stop everything on Windows 10 that you don’t it will be faster, way faster than anything running GNOME.
The problem isn’t the OS per si, the problem is the UI. GNOME is SLOW as hell and even if the OS behind it is way more efficient than Windows it will lose against a debloated Windows 10 setup because Window’s UI is fully native and way faster.