does mixing Java and Kotlin files in the same JVM project affect build speeds negatively? Is it more efficient to be 100% Kotlin? (note: this applies to a JVM project that imports JARs/AARs for Android)

iirc yes, but I can’t find source 😕 But I recall some performance hit related to multiple compilation passes so that Java sources know about Kotlin classes and Kotlin sources know about Java classes

Yes, Java compilation is a separate call to the Java compiler.