You need to design a web game around the download bandwidth, and only stream data right when it's needed instead of preloading everything upfront. Porting exist

You need to design a web game around the download bandwidth, and only stream data right when it's needed instead of preloading everything upfront. Porting existing games with a different streaming model usually doesn't make much sense. And once you build a web game from the ground up anyway, there's little reason to use one of the humongous game engines like Unity or Unreal which have been designed for a very different data streaming approach.