We've been using it since 1.0 and I agree with you that there's many rough edges... many projects may never run into them, but like you, we did have these issues you mention... slower performance by the compiler and IDE are very, very noticeable on a large Kotlin project (at runtime, we haven't had many issues - there's only a few cases Kotlin is slower than Java, but you can keep those in mind and avoid them when necessary[1])... one more issue is Dokka (the equivalent to Javadocs for Kotlin). It's still in alpha and it's pretty terrible, if I may say... it generates documentation that requires lots of JS libraries to run, which is just ridiculous. We ended up writing the documentation conversion ourselves instead of using Dokka (this was actually for an Android project - because we hadn't needed to publish docs for backend Kotlin before).
That said: we still enjoy using Kotlin, I just wouldn't ditch Java for it if I had to pick one.
[1] https://sites.google.com/a/athaydes.com/renato-athaydes/post...