Slow scala compile when upgrading from gradle 5 to 6


we’ve just done an upgrade from Java 8 and Gradle 5.5.1 to Java 11 and Gradle 6.0.1 for our scala project, and the compile time on our CI increased 6 times (from ~3 minutes to ~18 minutes).
Then we tried switching to Gradle 5.6.4 and Java 11 to see if Java 11 might be the culprit, but everything is ok with 5.6.4

Looking at scan output we see that org.gradle.api.tasks.scala.ScalaCompile execution task increased dramatically:

Execution time samples, Gradle 6, Java 11:

:service-hydra:compileScala 1m 5.121s	
:service-core:core-planner:compileScala 51.113s	
:service-b2b:b2b-main:compileScala 45.978s	
:service-vault:vault-main:compileScala 38.816s
:service-core:core-main:compileScala 37.865s

Execution time samples, Gradle 5, Java 11:

:service-hydra:compileScala 36.529s
:service-core:core-planner:compileScala 8.632s
:service-b2b:b2b-main:compileScala 7.937s
:service-vault:vault-main:compileScala 6.258s	
:service-core:core-main:compileScala 6.091s	

Playing with options.fork, --parallel, -Xmx, --max-workers settings we managed to get build time down to around 9 minutes.

Any hints?