The lag that is eliminated is the local echo of characters as you type, as mosh is sending those asynchronously in the background. If you have fancy, more interactive prompt/cmdline on the remote server then you'll still see the issue.
Ditto if you run tmux on the remote server. I really wanted to like/use mosh, but there's too many issues of reality that it can't gloss over.
If you're just maintaining basic SSH sessions into remote servers, then it's great. But if you have a nice, interactive remote development environment then you still get the same lag and latency issues.