From the JavaDoc: “Any task that has custom actions (i.e. ones added via {@link org.gradle.api.Task#doLast(org.gradle.api.Action)} or {@link org.gradle.api.Task#doFirst(org.gradle.api.Action)}) is not considered parallelizable”
There are some good use cases that require doFirst/doLast on existing tasks. For instance: If you want to do bytecode enhancement without breaking up-to-date-checking on the classes folder, you can add a doLast block to the JavaCompile task. It would be a pity if that resulted in serial execution of Java compilation.
I think it would be useful to have doLast(org.gradle.api.Action, boolean parallelizable) in order to get around this limitiation.