Browsers isolate each tab in its own process for security and stability. That is why one crashed page no longer takes down the whole browser — but it also means each tab carries its own memory cost.
Modern pages are applications. A background tab may keep polling for messages, playing hidden media, running analytics or holding a large data set in memory long after you last looked at it.
When memory runs short, the operating system starts moving data to disk, and the slowdown spreads to everything else on the machine, not just the browser.
Browsers now suspend inactive tabs automatically, but pinned tabs, media players and pages with unsaved input are usually exempt.