"This is true for the very first expression. That's because it is downloading packages and building everything. After that, it has all that information, so it's faster." - https://github.com/elm-lang/elm-repl/issues/128#issuecomment...
And if you have no internet connection or the package server is down, the REPL will not evaluate any expressions at all, it just shows an HTTP error message on every input (after a timeout).
The only option for offline usage seems to be running the REPL inside a configured elm project directory which has had the core packages downloaded/installed: https://github.com/elm-lang/elm-repl/issues/54