When incrementally building an exploded image, the ModulePackages attribute sometimes goes missing from module-info.class. This can have a severe impact on startup times of the exploded jdk:

make clean && make jdk
time build/linux-x64/jdk/bin/java -version
...
real 0m0.553s
user 0m1.732s
sys 0m0.088s

<edit some file in src/java.base/share/classes/java/lang>
make jdk
time build/linux-x64/jdk/bin/java -version
...
real 0m0.967s
user 0m5.032s
sys 0m0.284s

Expected:
build/linux-x64/jdk/bin/javap -p -v build/linux-x64/jdk/modules/java.base/module-info.class | grep ModulePackages
  #496 = Utf8 ModulePackages
ModulePackages:

Actual:
No output on an incremental build where java.base was modified. Degradation in startup times.

(Severity seems to depend on how many modules you touch before rebuilding.)