In python and node it is _so_ painful to use multiple cores, whereas in .net you have parallel for loops and Task.WhenAll for over a decade. Java is similar in this sense that you don't have to do anything to use multiple cores and can just run multiple tasks without having to worry about passing state etc between 'workers'.
This actually becomes a really big problem for web performance, something I'm deeply passionate about. Not everything is just IO driven holdups, sometimes you do need to use a fair bit of CPU to solve a problem, and when you can't do it in parallel easily it ends up causing a lot of UX issues.
Gradle with Kotlin DSL is nice, what's annoying is Gradle's constant API reshuffling for the sake of it that breaks plugins. Some plugins also introduce pointless breaking changes just to have a fancier DSL.
The IDE support is not an issue in practice, in my opinion, because IDEA is the best IDE for both Java and Kotlin. The official Kotlin LSP was released 6 months ago, but I haven't tried it.
