The feature is called Tab Unloading, and weirdly enough they made it not easy to access despite its usefulness.
You basically have to type about:unloads in the address bar and hit enter. If you then click on “Unload”, it will put the least used tabs to sleep. If you keep clicking that button until it’s greyed out, you’ll have unloaded all your tabs from memory.
This feature is handy if you want to temporarily switch to something that is memory hungry without having to close your 100 tabs.
For this you have to know that what gets swapped to disk is not the static content that you’d load upon opening a website, it’s the entire memory used by the tab.
Static web content is usually kilobytes to megabytes and is also largely cached (on disk even). A tab’s memory usage OTOH ranges from dozens to hundreds of MB.
Even a fast drive needs quite a long time (in computer terms) to load something like that, especially given that the access is likely not sequential and has a low queue depth.