Consider that people pay a $300 premium to get ~10% better performance (buying an RTX 5080 instead of a 5070 Ti).
Personally I know that sometimes closing the web browser in the background makes my game run better - that web browser doesn't even interact with the game! Would a sandbox have a smaller impact?
Buying a better GPU improves your graphics performance and that's basically unrelated to the area where a sandbox impacts performance.
Killing your web browser is probably just lowering memory pressure?
Sandboxes add overhead to syscalls. It's kind of similar to running under Wine, which also adds significant syscalls overhead. Wine also has a much more impactful DirectX translation layer, so your sandbox performance would be probably be much better than the Wine performance.
That’s hard to believe, given that many games run better under WINE than native Windows.
On Linux certainly so, and I think if Steam is installed as a flatpak all games naturally are sandboxed.