Co-routines have been in Kotlin for a while, got added to Java (Project loom) recently. Same for startup time. Native image support in Project Leyden
Downsides are that highly dynamic and reflection-happy Java code bases/libs won't run OOTB, that using profile-guided optimization (for achieving or surpassing HotSpot perfornance levels) requires a commercial EE license from Oracle (about 300-400 bucks per seat last I checked, not required for mere development until used in prod or at customer sites, and bundled with commercial JRE/JDK subscription since about a year), that even with EE your garbage collector choices are limited, and the somewhat overwhelming number of optimization flags to pass to native-image, not unlike earlier JVM runtime arg excesses.