[kevingadd]

This is also just because APIs like Object.defineProperty are old and well-optimized, while Proxy (in comparison - it's not THAT new) is relatively new and not optimized because it is rarely used. Part of the reason it's rarely used is that it's very slow :-)

[maxloh]

That seems like a loop.

It is slow it is rarely used it doesn't get optimized it is slow

[kevingadd]

That is generally how web platform optimization goes, yes. Browser vendors optimize the stuff that gets used most, which means new APIs tend not to get used much unless they fill a gaping hole.