This needs way more context. In general C++ is slower to develop and harder to maintain if you don't already have dedicated C++ developers and your code is being developed in C++.
Usually the best thing to do is to build a working prototype with tools that are most efficient in building working prototypes. If your idea doesn't do the job you save a lot of time and money. If it does the job really well you will have a lot of pain due to the technical debt that you acquired but you will also a lot of money to pay for the pain.
In conclusion: I'm 99% sure that you're asking the wrong question.
WASM code (regardless of the language), will usually be faster if it is mostly compute and little DOM interactions, as doing that requires going through JavaScript anyway.
And if doing 3D stuff, you're better off doing performance critical stuff on shaders, powered by the GPU, than dealing with C++ targeting WebAssembly.
Usually the best thing to do is to build a working prototype with tools that are most efficient in building working prototypes. If your idea doesn't do the job you save a lot of time and money. If it does the job really well you will have a lot of pain due to the technical debt that you acquired but you will also a lot of money to pay for the pain.
In conclusion: I'm 99% sure that you're asking the wrong question.