When compile times become a problem, how can Scala developers reason about the relation between their code and compile times?

Add the scalac-profiling compiler plugin into your project:

addCompilerPlugin("ch.epfl.scala" %% "scalac-profiling" % "<version>" cross CrossVersion.full)

Note that in Scala 2.13, the preferred form of the compiler option to enable statistics is -Vstatistics. It is part of the family of -V flags that enable various "verbose" behaviors (in 2.12, the flag is called -Ystatistics).

Learn more at https://scalacenter.github.io/scalac-profiling.

Also, you may wish to read the Speeding Up Compilation Time with scalac-profiling in the scala-lang blog. Worth noting that the article is 5+ years old, and hasn't been updated. But still, you may gather a lot of ideas while reading it.

This tool was created at the Scala Center in 2017 and 2018 as the result of the proposal SCP-10, submitted by a corporate member of the board. The Center is seeking new corporate members to fund activities such as these, to benefit the entire Scala community.

The plugin is now community-maintained, with maintenance overseen by the Center. Thanks to volunteer contributors, the latest release 1.1.0 supports both Scala 2.13 and 2.12.

The historical context of this project is quite interesting. For those wondering about the details, see the dedicated section.