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I understand but isn't Wren supposed to be a new language being developed? since it is still being developed it could be customized and tweaked to meet any requirements - or what do you think?

even for established languages, there are always new versions released to be more efficient

I don't know much about either language to be honest, but suspect that since webassembly has much more support its performance will be faster, and they probably need to squeeze every bit out of their platform quicker than they could by waiting for wren to mature.

A few languages can compile to down to webassembly though.