I'm happy to see you are trying to add Kotlin support. I'm waiting for it so much !
I'm not as you said 'very familiar with Kotlin environment, compilation & configuration stuff, etc...", but as an IntelliJ IDEA user, I noticed "Keep compiler process alive between invocation" and "Enable precise incremental compilation" can speed up build time quite significantly (7s to 2.5s for an Hello world !). I haven't found any documentation that explains how to use these options from kotlinc command-line compiler.
However, if you look at the Kotlin IDEA plugin source code, you can found the three binded java options :
Maybe you could give it a try.
Please keep us informed of your progress !