Under JDK-8150641, performance of repeated compilation in JDK 9 was significantly improved. But it is still slower than in JDK 8, using the speedtest from JDK-8150641, using NetBeans 8.0 installation as the source of jar files:
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SpeedTest.java-empty-shared
JDK 8
average compilation time: 7.1
vanilla JDK9
average compilation time: 24.975
SpeedTest.java-empty-unshared
JDK 8
average compilation time: 9.15
vanilla JDK9
average compilation time: 26.575
SpeedTest.java-jars-shared
JDK 8
average compilation time: 12.975
vanilla JDK9
average compilation time: 48.7
SpeedTest.java-jars-unshared
JDK 8
average compilation time: 133.6
vanilla JDK9
average compilation time: 353.175
---