If the running code is short enough then that copy might easily make the wasm version much slower. That is indeed a known downside of wasm (calls to JS are somewhat slow, and copying of data even more so - wasm shines when you can avoid those things, which certainly limits where it makes sense!).
If it's not that, then a 10x difference suggests you are running into some kind of a VM bug or limitation.
https://github.com/zandaqo/iswasmfast/blob/54bbb7b539c127185...
If the running code is short enough then that copy might easily make the wasm version much slower. That is indeed a known downside of wasm (calls to JS are somewhat slow, and copying of data even more so - wasm shines when you can avoid those things, which certainly limits where it makes sense!).
If it's not that, then a 10x difference suggests you are running into some kind of a VM bug or limitation.