[regularfry]

If it's cached to the filesystem, it'll be handled by the file cache, under the kernel's control with the rest of RAM. Maybe browser makers have a good reason for thinking they can do better than the OS, I don't know, but having two systems trying to do the same job with the same resources sounds like a recipe for instability and inefficiency to me.

[scott_s]

If you have to cross the kernel boundary every time you want to access something in your cache, your "cache" is now much, much slower. Note that this applies even if the OS keeps the file in memory, and doesn't require going to disk.

[regularfry]

Sure, but if accepting that cache slowdown makes the rest of the system more responsive and more useful for background tasks, it may be worthwhile. It's a trade-off, and the browser makers have every incentive to be as selfish as possible.