Saying Groovy is 'slower' is a bit of bullshit. The slowness of the language depends on the developer.

Besides, when Gradle switched to Kotlin (from Groovy) recently, they found it ran SLOWER on Kotlin https://youtrack.jetbrains.com/issue/KT-24668/Gradle-Kotlin-DSL-is-3-4-times-slower-than-Groovy-one-on-first-use-on-many-simple-projects-build-benchmark