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.
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.
No. The JavaScript code will be faster.