It varies. Here's an example of some performance analysis I did on the expat port: https://bugzilla.mozilla.org/show_bug.cgi?id=1688452#c37

There are detailed performance numbers here on a variety of real-world codebases:

https://kripken.github.io/blog/wasm/2020/07/27/wasmboxc.html

tl;dr Something like ~14% when using the best bounds checking strategy, or ~42% when using the most portable one. (There are options in the middle as well.)