SBT is slow because compiles internal code that is done in Scala and Scala compilation is slow because is a complex language (but once Scala is compiled is a lot faster at runtime)
You can give SBT a boost when using SBT 1.x version with the SBT server. The SBT server allows you to use just one SBT instance shared between command line and the IDE. This is pretty useful, more info here: https://www.scala-sbt.org/1.x/docs/sbt-server.html
Also considere using other build tools that are lighter, like PANTS, that is based on Python which is interpreted and a lot faster. More info at: https://www.pantsbuild.org/
NOTE: the PANTS documentation and community is not as extensive as with SBT but is worth the try, there are amazing things that can be done with PANTS.
NOTE2: if your code base is large it will still take a lot of time to compile/build, so considere to arrange your code and artifacts as incremental/cached pieces/subprojects to see a real boost.