Using scalac is not a standard use case.

Standard use case is to use sbt (well, or mill, since we're in lihaoyi's thread :D). Sbt starts up slow, but with bloop, or sbtn (native client of sbt), the compilation is really fast (much faster than C++ for example):

    $ time sbtc compile                                                                          
    [info] entering *experimental* thin client - BEEP WHIRR
    [info] terminate the server with `shutdown`
    > compile
    [info] compiling 1 Scala source to /home/.../target/scala-2.13/classes ...
    [info] compile completed
    [success] Total time: 0 s, completed Feb 11, 2021 4:21:51 PM
    sbtc compile  0,08s user 0,02s system 21% cpu 0,492 total