A modern browser runs each tab in its own isolated process for safety. That isolation is good for stability and bad for memory, because every tab carries its own overhead.
A page is not just text. It holds images, scripts, video players and live connections, and many sites keep working in the background after you switch away from them.
When memory runs out, the system starts moving less-used data to the drive. That swap is far slower than memory, which is why the whole machine — not just the browser — becomes sluggish.
The result is a cliff rather than a slope. Everything feels fine until memory fills, then every action pauses while data shuttles to and from disk.