Calling a C function with Cgo has a certain overhead if you compare it to calling a Go function. But cgo overheads are completely negligible as soon as the function does any kind of real work. (A bigger reason to avoid cgo would be if you want easier cross-compilation.)
cgo execution overhead is a nothingburger, but the programmer overhead to convert between Go types and C->Javascript types, and vice versa, as required by the use of a third-party engine (v8, etc.) is still significant as compared to what Goja offers. That is what the parent is talking about.
