Emsripten technology with Amazon and Google

regarding for this subject page

I see two companies (amazon by firfox and google by chrome) are used emscripten technology to run c/c++ code on web by converted to javascripe

there are different SDK for both, Native Client SDK by chrome and emscripten sdk by firefox

firefox link

chrome link

I need to know which is fast SDK ( Amazon emscripten sdk or Google Native Client sdk)?

and why Unreal engine 4 wasn’t used Native Client-sdk as on chrome native client sdk?

the last…
if I need to used custom unreal build tool (UBT) files for architecture study, where I can start and how I can got that files to used on own separate project.

Google NaCl not compile c++ code to js.