No, it won't be faster, if you only optimize a lesser language. If you have higher level code running your optimizer can do more than if it only has a low level version.
No, it won't be more secure. Js0 might be more secure, but if we sites all run any of dozens of different tools those tools are going to be creating the vulnerabilities. It's shifting where security issues occur, and creating more of them.
I'm terrified this could happen. JS has gotten so much better over time. We are so close to being able to not need transpilers. This sounds like such an absurd cop out for browsers to say, meh, we just don't want to do the work to implement. Being so close & then saying, sorry, you must use big toolchains to develop for the web is a monstrously bad future.