The thing is that if you want to compile a single fmt.Println("Hello") line, that will produce a large output because of what's needed to support it, including

The thing is that if you want to compile a single fmt.Println("Hello") line, that will produce a large output because of what's needed to support it, including dealing with Go/JS types like bytes, runes, slices, standard library ("fmt" and things it imports), etc. That part is constant. Once you write more code, the generated output will grow very gracefully as expected. The output is quite large (order of 1~2 MB), even when minified, so it might not work well yet for user facing sites with huge traffic. That could be an ongoing improvement effort for GopherJS to benefit from. However, for smaller sites or offline utilities it works absolutely great.